Fresh cum on your face can be fun sometimes and clears up your skin, but once the encounter is over it starts itching and feels gross as it dries. That's when you demand a towel and go wash up before bed.
The "saving my ass for my wedding night" is a new one for me. Interesting. But DON'T fall for it. I had a similar situation with my ex-wife. She never made such a statement, but she did promise to give up the ass if I married her and did something else for her. I did both and she reneged on her end of the bargain.
More useful advice for getting a girl to "give up her ass" would be to ease her into it: Explain that it can enhance her pleasure, watch anal porn with her, let her use a small dildo on herself with plenty of lube, have her wear a butt plug during vaginal, and let her get on top the first time so she doesn't have to worry about it hurting.
I did mention she is my EX-wife.