Simply - there is no healthy relationship with two people who belong together that can participate in a threesome. That might be a broad generalization, but like ivory soap, it's damn near pure.

That being said - you're a young kid. You probably won't end up with this girl. So git 'er done (literally). I've had many a relationship that I thought was forever where 6 months or a year down the road I realized I was an idiot. That'll happen until you're 25 or so. In the meantime, get as much done as you can.

CSP