An important part of the therapy is "reality testing".

You might say, "I'm terrified of taking a girl to bed cos she'll probably just mock me for my tiny penis".

Well she might. Maybe one already did. So then the therapist might say, "Okay, 3 girls laughed, does that mean *all* girls will laugh? Maybe they will. There are at least 500 million eligible girls on earth and it's possible they'll all laugh. Three have laughed so far... but is that really enough to estimate that they *all* will? It seems a bit few to me. How about 50? How about you take another 47 girls to bed and if they all laugh, then that'll make it more likely that 'all' girls will laugh."

And then they all laugh up to girl number 6 who also laughs, but then says she really likes you and wants to see you again. What now? Is laughter a bad thing? Maybe they're just amused and like laughing.

So negative thoughts are often "right" but maybe they're more nuanced than you're currently seeing.

Or maybe the girls are being horrible to you. What kind of girls are you going for? Only bitchy "cool" girls? Oh okay, so maybe try 47 other women of different types and see if they treat you differently. Try a goth, a metalhead, a cheerleader, a librarian type... "reality test" all the way to self-comfort. :D