It sounds like he has theories based on religious warnings of loose women. You can't cure stupid and you shouldn't sleep with stupid. Kegel exercises are good for the area though.
Just one argument, After delivery of a child, vagina returns to almost normal size.....Yes he may lose his libido, if he believes such theory.
If he is accusing you of cheating on him for such a ridiculously convoluted and paranoid delusion then dump him because he's not smart enough to be in your life. You'll be so much better off without someone like that in your life.
Correct. But if "he" refuses to believe the Internet, confirmation from a bunch of people on MedHelp won't make any difference to him.