Re: My 17 year old daughter and her thongs..
This is one of those times when I think you have to pick your battles. Considering her age and relationship status, and that she is responsible in other ways, I would let this one go.
By now she should be doing her own laundry anyway, and you should not have to see them if they bother you. I see lots of young women wearing them, in good taste.
Time to let go of the little stuff, as hard as it is to do so.