<human>: To preface the story a little bit, my girlfriend was raised in a very conservative home and has NEVER had a serious relationship. Her parents are VERY conservative. They claim that they never slept with each other until they were married, which is believable. I would consider myself a "nice" guy, I have dated and had sex with girls in the past but never treated them badly.

I don't see anything wrong with that but my current girlfriend and I have agreed NOT to have sex until we're ready/married.

THE SITUATION

We've been dating for about 3 months and we occassionally spend the night at each other's houses. One night she was wearing a dress so she borrowed a shirt and some gym shorts to sleep in. The next day she wore them home and her mom saw them in her laundry (somehow she just knew they were mine). Her mom didn't say anything about it but she made a very "suspicious" face towards my girlfriend. She's very close with her parents and knows that they will want to talk about it next time she comes home.... 

I'm very open with my parents and tell them everything that I do. So when my girlfriend asks me, "What am I supposed to tell my parents?!", I don't have much to offer.... *Please advise

Provide a brief overview of the text.
<bot>: Conservative parents know their daughter has spent the night at my house (no sex happened) and they have a problem with it. What should she say when they bring it up?