Everything in life is a risk: just a case of weighing odds really.
HIV: incredibly unlikely but fairly major if it happens so worth being at least concerned enough about always to use condoms. (I've just read that some desperate prossies are taking antivirals instead. As if they weren't fucked up enough already.) HIV is far less infectious than gonorrhea, syphilis, and chlamydia, so if you wear confoms it shouldn't be a big worry.
Gonny n syph: slight chance of the former, my guess would be once every 700 prossies when using condoms and even then almost entirely from OWO (it's far more infectious than many other infectious stds). Normally shows symptoms in two weeks so, if worried, two weeks of abstinence before doing the wife is a reasonable precaution Of course, you might be one of the rare unlucky bastards that is the 1 in 700 and the one in ten of those (ie 1 in 7000) that shows no symptoms. Possible to treat yourself in emergency with antibiotics, but if you've been taking antibiotics for something else they might mask it, meaning you wouldn't know you'd caught it. Syph is much much rarer.
Chlamydia: very common, often goes unrecognised so no easy 'safety' period. Just wear condoms to avoid it as much as possible. Symptoms can take ages to show to who can say where you got it. Anyone with a normal sex life could pick it up and not know for ages. Ditto herpes of course.
Thrush: which is not sexually transmitted but incredibly common and a bit unpleasant, especially for women. It can be brought on by a lot of friction, for instance. Or rubbish diet or being under the weather. Or tampons: which won't affect men, but shagging someone with thrush increases the possibility that you get it, and even bring on thrush in her without even knowing it. So, :dash:as with gonny, a safety period is one idea. Another, if you've been shagging five different funny-smelling whores a day, is just to treat yourself : as a safeguard if you feel itchy. This is not accepted medical advice: just practical, for those that know what they're doing. A tube of canestan (no prescription needed) from the chemist, used for a couple of days, and you're good to go! In terms of not messing up your wifey's fanny, I'd say thrush is one of the commonest but easiest to sort out. (Millions of women do so every day.) This assumes you are a caring fucker, rather than saying, "Ah, she coulda got it anyway! Musta been all that tight designer underwear and all the other stuff." Be nice.
For those who are a-wondering, if you need antibiotics you can get them from the same place you probably get your overseas generic viag. Or if you are too illiterate to get the type and dose right (probably like James) then just go to the fucking GUM clinic ffs ... like a million other fuckers!
Happy punting, and if your ball n chain wants a wee shag occasionally, you can now consider being a gent and giving her one, without making her fanny glow in the dark or killing her off with a longterm dose of syph!
(Any royalties earned by Mumsnet after stealing this post that was not meant for them, can be placed in an unused condom and mailed. Packaging will be returned covered in the juices of a first class atteactive young whore who, unlike the scabrous creatures that haunt that site, had fuck all wrong with her juices or ability to satisfy a nice thick pork chop.