Like everyone has said, if in a monogomaous relationship, it's understandable that two people might not use protection. Any other relationship (booty calls, casual sex, one night stands) is entirely on your shoulders. If you don't have a commitment with someone, then how can you expect them to lay it all on the table for you? If you are married and get infected by your spouse, that's different. There is supposed to be an openness that allows two people to trust eachother 100%. Sometimes one partner doesn't see it that way and it affects the other one.