Eh, I tried looking stuff up, couldn't find much of anything. So I come here, on IA (****ing hell... )

anyways, odd issue - anytime I put a condom on for doing the dirty, unless I'm like doing missionary, but say if the girl is on top, I seem to just lose my happiness and go flaccid, and at times even when it is missionary. Granted the girl I've been seeing wants me without it ( she's on the pill etc ) and without the condom on I'm just peachy king. Someone mentioned that I might need a larger sized condom ( girth wise ) that it might be too constrictive, but I don't know? Is it because I'm still just not used to condoms, or should I go ahead and try a larger girth condom?

Odd question to ask, but eh, it bothers me to some extent.