Right now your preschooler is learning to pretend, and part of this process is experimenting with imaginary roles. Playing dress-up — whether he wants to look like Daddy, Mommy, a football player, or a fairy princess — is common and entirely normal at this age, and isn't limited to roles embodied by members of a child's own sex. (It's interesting that parents are often concerned about boys who enjoy girls' toys or clothes but don't seem to mind when girls play with trucks or dress up like cowboys.)
The fact that your son enjoys playing with girls' things is an indication that you've been open and supportive and that you've provided him with opportunities that go beyond typical "boys'" play — and not an indication of his future sexual preference. The truth is, he may grow up to be gay or he may not, but playing with Barbies at age 2 or 3 isn't going to "make" him anything other than an imaginative child.
To further encourage your child's normal and healthy imagination, make sure he has a chance to play with a variety of toys in a variety of ways. Also be alert for gender stereotyping in the stories you read together, the activities you provide, and even the offhand comments you make. Rather than "shushing" your child when he tears up, for instance, let him know that crying is okay. Be sure to encourage his gentleness with pets or younger siblings, too.
Of course, even if you're open-minded about your son's experimentation, friends, family members, and even complete strangers may not be. Respectfully acknowledge their comments (or defuse them with humor), but also remind yourself to trust and value your own judgment. After all, if your child is happy and playful — even if he's trying Mom's high heels on for size — then you're doing a good job.