Answer: First, there is no law that says you have to have vaginal intercourse with a male partner. It is perfectly okay so say no to intercourse. In fact, this is something I mention on the pages about Virginity. At nineteen, most women probably should be saying no to intercourse; because of the risk of pregnancy. This is a totally separate issue from your sexual orientation.
It sounds like you are bisexual. That is, you are sexually attracted to men and women. You can be bisexual and still have a preference for one sex or the other. You can have a primary relationship with a man or woman and a secondary relationship with a man or woman.
We are all born bisexual. If your mother breast-fed you, you likely became sexually aroused when you nursed; as well as your mother. You likely enjoyed it when either parent washed or put lotion on your vulva. Your first childhood explorative/sexual partners were possibly boys and girls. During puberty you were likely curious about your girlfriend's changing bodies, as well as that of your male friends. We can enjoy the physical stimulation either sex can provide us, it is more a matter of which sex(es) we are attracted to.
Sexual orientation is a complex subject. I believe a person's sexual orientation is dependent on two main factors, biological sex drive and emotional intimacy.
If you are sexually attracted to someone, his or her presence should at times cause a physical response, sexual arousal. This is triggered by their mere presence, not sexual thoughts etc. Your heart rate increases, you feel funny, your vagina lubricates, your clitoris fills with blood, and your genitals become warm and moist. This is something that happens automatically and that you have no control over.
Sexual attraction is the easy part. Being attracted to someone gains you nothing if you cannot have a trusting and intimate emotional relationship with them. If you cannot love the people you are attracted to, your going to have problematic relationships.
You have told your boyfriend you may never have intercourse with him. He needs to accept that or go elsewhere. He may be perfectly happy not having intercourse. Maybe he isn't ready either. As long as you clearly define what it is you want, any problems that arises are his, not yours. You can certainly define what you want in a relationship. He needs to accept you for who you are, not who he wants you to be. Do not try to be someone you are not. Will he be as excepting of your bisexuality, and preference for women, down the road, probably not, but there really isn't anything you can do about this. You will have to cross that bridge when you reach it.
I also believe a person's sexual orientation is not fixed in stone. Your sexual orientation will depend on your needs at any given time, and who is available to fulfill them. Being a lesbian gains you nothing if there are no available female partners. Should a woman in this situation go without or find a male partner? I am aware of a woman who identified as heterosexual and got married, divorced and identified as lesbian, only to later marry a man again. I am aware of a lesbian who secretly had male sexual partners. We live a long time, our needs change with time.
You are young and need to figure out what your needs are. Do not be too eager or allow yourself to be pressured into being someone you are not. Both the heterosexual and lesbians communities often express an intolerance of bisexual women, saying they are women who are unwilling to commit. If you like both men and women, you like men and women. Do not accept a label you are not comfortable with. Perhaps you are not bisexual, lesbian, or heterosexual, you are simply curious.
I do not recommend a threesome until you have developed your sexual skills with men and women separately. If you are bisexual, you are not cheating on your male partner when you have sex with a woman, and vise versa. Your male/female relationships will likely be different from your female/female relationships. You can be committed to and love two people of opposite sexes, at the same time. You likely love your mother and father, for example.
In closing, I would like to say you sound perfectly "normal" and the situation you are facing is not uncommon for men and women, yet still confusing. You are a young woman who is exploring the world around her, and within herself.