Let me say right up front, that this post is not meant to be "preachy". Whilst I normally practice "safe sex", there have been the occasional times with one or two of my very special regular ladies, when we have got a bit "carried away" in the passion of the moment, and I have no regrets about that.
But I do want to high-light the realities of STDs, but in a balanced "risk" based assessment.
An Historical Perspective
First up, STD are not a new thing (though the deadly HIV virus is relatively new). During the two world-wars, military hospitals treated more VD (as it was known then) cases than battle injuries. In OZ immediately after WW-I there was an epidemic of syphilis. Returned servicemen picked up the "clap" in the brothels of Egypt, Palestine and France, returned home and infected their wives and subsequently their new-born children. It is estimated that around 1920, 10% of Australian babies were born with syphilitic conditions. This STD epidemic did not get publicity because, (a) such things were not discussed, and (b) it was over-shadowed by the influenza pandemic.
WW-II brought penicillin and other antibiotics and VD became a "mild", treatable annoyance.
Now I won't pursue the historical line, but rather I want to focus on the epidemiology of the situation. We are talking about contagious diseases spread through inter-personal (sexual) contact.
WHO - The Risk Network
Epidemiologists talk about the "Risk Network" of who has had contact with who. In this case, who have your partners had sex with and who have your partner's partners had sex with, etc. It is the "six degrees of separation" scenario. The following diagram from the California Department of Health Services maps the sexual inter-relationship network at a college in Colorado Springs where an outbreak of gonorrhea occurred in the 1980s'.
The majority of students only had 1 or 2 partners, but you can clearly see that there are about 5 individuals who were highly sexually active and most students were only 2 or 3 couplings away from these focii of infection.
HOW - Mechanisms of Contagion
The next aspect of epidemiology is the mechanism of infection. With STDs, we are talking about blood-born viruses so infection is essentially through blood-blood contact. Thus shared syringes and open wounds are the highest risks. But the lining of the vagina, penis and urethra have millions of blood capillaries very close to the surface of the skin and trans-cutaneous (through the skin) infection is highly likely. The viral load in saliva is effectively non-existent - the chief risk of oral infection is via cold-sores, ulcers, etc.
RISK MITIGATION
Barriers to the "How"
The condom is still the most effective method of avoiding contagion. With the range of thicknesses and sizes (girth), with some experimentation, you should be able to minimise any discomfort and loss of sensitivity. But correct use is still essential (fitting it on and withdrawal immediately on completion to avoid slipping off).
The question of barriers for oral (condoms for fellatio and dams for cunnilingus) is open to debate. The medics at my STD clinic tell me the risks from uncovered oral are minimal when there are no skin lesions on either partner.
Minimise Contagion "Entry Points"
In mentioning lesions, remember that contagious transfer can still occur in the uncovered pubic areas if there are warts, pimples, cuts, etc. Similarly with lip and mouth ulcers, cold sours etc. At home, we avoid kissing if either of us have a cold. Show the same respect to your sexual partners. In sport we now have the "blood rule". Apply the same rule to your sexual activity - if you (or your partner) have any open sores, then avoid intimate contact.
This is probably a good place to recommend you discover, explore and enjoy the full gamut of intimate interactions. A guy is much more likely to "get into trouble" if he is only focused on the one outcome, of "getting his rocks off" as quickly as possible. Also, having sex when drunk is also dangerous. A little liquor might help relax you, but with too much you quickly loose inhibition required for rational choice and risk minimization.
Choice of Partner - the "Who"
This very much falls into the risk minimization category rather than prevention. Historically, prostitutes have been the principal source of infection. But in recent decades, especially in countries like Australia with regulated prostitution, this situation has completely reversed. Recent surveys (in OZ) have shown that regulated prostitutes have the lowest incident of STD of any sexually active demographic. In STD clinics, infected men are 5 times more likely to have caught it from a casual encounter (pickup at the pub/club?) than from a prostitute. Since regulation, there have been zero cases in HIV identified among regulated/tested prostitutes. As one wag put it, professional electricians rarely get electrocuted - it is the weekend amateur that is in greatest risk. As I have written elsewhere, under Australian regulations, prostitutes are required to always practice "safe sex" (I would guess that we have better than 95% compliance) and must have monthly STD health checks (as good as 100% compliance with brothels records checked at random). Most perform a visual check of potential clients and any suspect conditions will have you turned away.
In whatever sort of "relationship", the key is communication and trust. And that is two way. Its not just "Can I trust her?", but "Can she trust me?". I mentioned health checks above - you do have checkups yourself I hope. Take your share of the responsibility.
Early Detection and Treatment
Generally, men are not good at caring about their own health. Surely your sexual health is as important or more-so than your general health. Get to know your own body - yes self examination applies to men too. In OZ we have free, anonymous sexual health clinics - use them. If we expect our Ladies of Pleasure to have monthly check-ups, we should show them the courtesy of having ourselves checked as well, say 6 or 12 monthly.
Like most diseases, the earlier anything is detected, the sooner treatment can start and the quicker you will recover.
The Bottom Line - Risk, the Numbers "Game"
Now I'm no professional in this area and I can't put numeric probabilities on various aspects of risk and mitigation. All of life is risky. Just keep things in proportion in your mind. There is no point in being paranoid about contracting HIV from a drop of semen on the skin when the risk of being run-over by a car when crossing the road outside is a ten thousand times higher.
Outside of a monogamous or closed polygamous relationship, I don't advocate 'au naturale' as your normal 'modus operandi'. But having followed all the above risk mitigations, I am comfortable in taking the risk of an occasional "in flagrante delicto".
Be Informed
A short post like this can't possibly cover all aspects of this topic. So read up further. If in any doubt, talk to a medical practitioner (use one of the free anonymous services). And feel free to add your informative comments, though I do have the power of moderation.