Girls say that I am true "boyfriend material" to take home to their parents. The problem is that when I tell them that I am gay, they don't believe me. I've known some of these girls from three to eleven years. I've never "experienced" but have dated and kissed girls. They think that they can change me and make me become straight. How can I force the fact in their minds that I am gay without hurting them?
You might hurt your friends' feelings if you call attention to their uninformed beliefs. Trying to "make" a gay person straight is as disrespectful an idea as making a straight person gay. Your friends' refusal to accept your sexual identity may be homophobic. Your gay identity is not something that is temporary or can be changed easily or readily. People who believe that it can be have the misinformed notion that homosexuality and bisexuality are choices selected by all who think of themselves in these terms.
Back to you, it sounds as though your friends are hurting you by refusing to accept you and how you identify. It's your job first to make sure you're respected, supported, and safe. If your "friends" cannot accept you for who you are then maybe it's time to think about letting them go in order to make room for others who will like you as you are right now. A real friend accepts the close people in their life for who they are and doesn't set out to change them.
Maybe these friends think of their comments as harmless jokes and don't realize how you really feel about them. It's up to you to make clear that you don't find it funny and, while thanking them for the compliments, you want them to stop. You can explain that your gay identity is real, and that their belief that they can change you and make you become straight is uncomfortable for you, embarrassing for them, and threatens the future of your friendships. Perhaps printing out copies of this Q&A and giving them to your friends might be a good way to start a conversation.
You mention that you've dated and kissed girls, but not had sex with them — assuming that this is what you meant by "experienced." Some gay men (or lesbian women) have kissed or even had sexual encounters with people of the opposite sex, especially during the periods of their lives when they were still coming to understand their sexual identity (similarly, many men and women who identify as straight also have kissed or been sexual with members of the same sex!). What you did in the past does not have to have any bearing on how you identify today. If you are interested in men, then that is that.
It sounds like you're quite a catch, and some person out there is going to be very lucky when they that you're "boyfriend material."Alice!