C.R.
Okay now, first of all, she NEVER at any point said she was worried about him being gay and I think its horrible that everyone just jumped to that conclusion. There could be any number of reasons she would prefer her son not play with only girl toys. As for 'nipping in the bud' she could just be thinking of how her child will be treated by other little kids if he shows up at kindergarten with a princess in his backpack, so give her a break.
S., I have a 3 year old boy who does love the cars and trains and such but his favorite color is pink :) He also wanted a baby doll when I was pregnant for his sister so I searched everywhere and was able to find a little boy baby doll at Target and got that for him. He was thrilled that I got him a boy one (although he did name is Stella like his sister) and he pretends to feed his stuffed animals bottles and swaddles them in washclothes. If I gave him a bunch of barbies he'd play with them too. He has a great imagination and I think the dolls/barbies are just a way for him to act of what he sees daily in his life. They are 'adults' he can control and they are probably a 'comfort toy' since he was used to playing with them with his cousin. I can understand how you feel though, I wouldn't mind getting maybe one barbie for Luca to play with but I wouldn't want my sweet little man to only want girl toys either :)