Prove yourself to regain her trust, it will not happen over night, time cures most problems.
Being 17 dating a 16 year old myself just let it go apologize,if she forgives you that's great if not,there's not much more you can do my stepmom hates my boyfriend for no apparent reason,sometimes there's just no changing that.
you should apologize to the mother and ask the mother if you could have a talk with her alone.ask her how you can prove your trust to her and what would it take ,and would she give you a chance.Her daughter has to be willing not to get you into trouble and to follow the mothers rules as well,good luck.
I don't agree with Northguy. Once you make the family mad it makes everybody uncomfortable. You need to convince her mother your intention is to care, love, and cherish her daughter, and mean it.
You're 22-25 years old...how old is your girlfriend? If she's the same age as you, then why should you care if her mom trusts you or not. You're both old enough to do whatever you want without mommy looming over your shoulder.
In that case, behave yourselves, you bundles of hormones! LOL. Just stay in the living room in plain view. That should relax mom a bit. Isn't it odd though, that your gf was laying with you, at her house, and yet it's YOU her mom doesn't trust? Parents.