There can be many reasons, including the ones noted above. A couple of years ago I started having this issue, so I asked my doctor (who happens to be gay) this question. It wasn't happening all the time only with unusually large dicks. In my case I had developed tiny internal hemorrhoids.
@BBB2.5 I have had both reactions. One guy stopped after a really long session, not really freaked. But I had a fuck buddy with an enormous cock, biggest I've ever seen, it didn't bother him at all. He said it happened frequently with both men and women because of his size. We fucked a lot and I only use white linens, used gallons of hydrogen peroxide so my bed didn't look like a crime scene