Tell your mom to stop crying, lesbians have children all the time.
You're focusing on the sex when you should be focusing on what you fantasize about when you think of love and marriage and who you'll be living with in that house with a picket fence. Only when we identify who we think that person is, do we find out what gender we happen to love. It's not about how your body feels, everyone has sexual fantasies. It's about how your mind feels, and your heart.
At 19, nobody has any business hauling out grandma's ring. Don't get married for at least 7 more years.
What Annie said. Give yourself time to figure out not just your sexual feelings, but your emotions as well. It's obvious that you have some inhibitions when it comes to your feelings about girls, and only time will allow you to sort things out. This is why getting married at your age would be a terrible idea, regardless of your orientation. You need time to mature emotionally, everyone does.
You also make it kind of sound like you have two different relationships going at once? Is everyone involved aware of this? There is nothing wrong with being polyamorous, but not being upfront about it still counts as cheating. You need to sort that out as well before moving forward, another reason why you're most likely not ready to accept that ring.
Either way, don't worry too much about how to label yourself, okay? Bi, lesbian, pan, poly... just be honest with your feelings and give yourself time, and you will find out what you're comfortable with. The words are just that. They can be useful in how to describe yourself, but they don't have to define everything you are. They're tools, not rules :)
I know this was posted a little while ago, but I still hope it can help!