(NSFW) What other reasons could be there for the swelling?

My boyfriend and I have been having sex for about two months and just recently, I especially noticed after we tried the ‘doggy style’ position that at the end of each session his private member always ends up being swollen and sometimes even has scratches on it.

We think it’s because I have a very tight lady part and that could be the reason, I’m always well lubricated so I don’t know what this could be.

Do you guys have any idea what it could be?