A lot of people assume that individuals with multiple, concurrent partners have the highest STD risk, but that isn't necessarily true. People who are aware they are at risk, and manage that risk, may actually be at lower risk that those individuals who assume their risk is lower because they're only sleeping with one person at a time... or sleeping with someone they know and trust. I bring this up because of a recent article in the Journal of Adolescent Research, which examined how many young adults have ongoing sexual relationships with their exes -- i.e. they keep sleeping with an ex-partner once they have broken up.
The study found that over half of young adults whose relationships broke up continued having sex with their exes. This is interesting to me, because it is exactly the sort of situation that I would consider to be very high risk. The two people in question know each other well enough that they aren't likely to be having ongoing conversations about safe sex and testing, but there's also a real chance that one or both of them has had other, potentially risky experiences outside of the dyad.
Now I want someone to do me a favor and study safe sex habits in "sex with an ex" relationships. I'm not currently in a position to do so myself, but I'm very curious about what the results would be!