Mary could still be a virgin!
Most viewers think that Mary had a "lost virginity" episode. The one where her parents come to visit and she didn't come home until 8:27 in the morning
There are so many things about that episode in particular that make it ambiguous. Even during the early 1970s, I'm sure many people were not ready for a single woman to be sleeping around like a harlot.
So I believe there are so many possibilities as to why Mary came home late. For one thing, notice she says she is going to a 'Dinner Party" . Its not quite the same as a "date".
Mary could have been a house guest with friends. The evening got late, and Mary didn't want to go home at such an odd hour, so her friends let her stay and sleep on the sofa. That could have happened! Or Mary simply decided to go out for a late night drink and she ended up not able to get home by herself, so she stayed there over night for her safety.
It doesn't mean she had s-e-x. Let's remember , Mary is still a good girl. I bet she was a virgin til the shows end.