Maybe you think it's not very often that you come in contact with the
other person's blood during sex. Well, it's not that simple. Even very
gentle sex causes tiny wounds, which can be in the mouth, the vagina or the anus, without you noticing.
You can also come into contact with blood, if the woman you're having sex with is on her period. No problem as long as there's nothing in the blood that can make you ill. But how can you know?
When blood flows during sex, whether intentionally or not, it
can contain disease-causing agents. Including such a dangerous one as
the AIDS causing HI-virus.
Even if your skin looks completely undamaged, you can still have minute
wounds – in your backside, on the head of your penis, in your mouth and
elsewhere (for instance, if you have a piercing somewhere). It's then
easy for pathogens to find their way into your body.
Apart from HIV, someone else's blood can pass on syphilis bacteria. Hepatitis-viruses type B and type C also occur in blood and can get into your body through wounds and scratches, no matter how small.
So be careful, especially when things get a bit rougher. Condoms
are a good form of protection – at least for your dick (or whatever
name you prefer for your penis). And for safety's sake, cover any
scratch or injury with a plaster.
Toys you use (or allow to be used on you) during sex should be clean and preferably disinfected.