You won't believe me but I have had sex with 9 women. Three in long term relationships, the rest casual.
Interestingly, a lot of the casual ones really enjoyed sex with me and said that I was the biggest they've ever had (unprompted)
But the three girl friends (who were generally speaking higher quality girls than the casual fucks) had all had bigger in the past and let it be known one way or the other.
I wouldn't say it's the only reason but it definitely played a part in all of those relationships not working out.
It's really insecure of me but I couldn't marry a girl unless I was the biggest she's had because I know for a fact that everything I typed in the OP is 100% true and so do you