It's not that uncommon a belief. My flatmate is str8, and he caught a couple of his work collegues laughing behind the back of a flamboyantly gay customer who had been in. He challenged them as to why they were making fun of the customer, and they made the dead brain reply of "Why? are you gay too?" He told that no, he wasn't, but his best friend was, and he'd hate to think that people made fun of me like that. They put two and two together and realised this was the same friend he shared a flat with, and started asking the most absurd questions.
Firstly, they asked why he wasn't afraid that I'd come into his bedroom in the middle of the night and rape him. When they heard I had a boyfriend, they were horrified that my flatmate might possibly hear us having sex through the walls.
They also said that it was a "Proven FactTM" that after a while of "being gay" that your ass breaks, and you have to wear tampons, and eventually a nappy. This always happens to all gays apparently.
Ignorance is everywhere. If you behave in the way that alex8's aquaintance does, and don't let your body recover after sex, and don't look after yourself, you may well through your negligence cause yourself problems. But they're an extreme. A friend of mine who's a doctor told me that the sphincter is a muscle, and even if it is stretched or weakened, it can be restored through simple pelvic exercises.