I need relationship advice... girlfriend wants sex but I'm not sure

when my girlfriend and I were 16 we spent an entire weekend for our 3rd anniversary at the beach… on the night of our anniversary, I gave her a promise ring and told her I wanted to commit to the relationship and wanted to marry her one day after school and college were done and over with… and I never once regretted it!

we are now 18 and have been living together on our own for 6 months since she turned 18 and are still virgins…

last night after dinner she came to me and sat on my lap and wanted to talk about having sex… and jokingly suggested we go to Vegas and elope but not tell our families so we’d technically be married… I told her I wanted to have sex with her too but didn’t want for her to have to choose between mothering a kid or education, because her education is important to her… and she said, “what are the chances of me getting pregnant if I take birth control and you wear a condom?” obviously she really wants it… and we’re practically married as it is (live in the same house, sleep in the same bed, both pay bills, both work and go to school (when it starts)) she said it was fine if I didn’t want to do it, that she’ll wait, but assured me she’ll be ready if I changed my mind… what should I do???

the reason I want to wait till marriage to do this is because of promises I made… not religious but because I promoised her adoptive mom that I would wait and her adoptive mom is hella protective over her… not that she’d ever find out, but she’s so close to her she’s come to know her BETTER than the back of her hand…