Hmmm ... It started out innocent enough, but she seems over-controlling and to constantly be upping the ante. In some contexts, her behavior would be defined as "grooming." If you're equally turned on, don't mind any of it and even like it, then there's no problem here. But if she's pushing you beyond your comfort levels, with promises of good things to come if you just do this one more thing for her, and if she keeps upping the ante, I see it as highly problematical.

I know a lot more about adults doing with children and men doing this with women. It seldom turns out well. She's plain lying when she says it won't go any further. It's constantly going further. This sounds more like domination and submission than a liking for lingerie and she's not being honest about it. If you're into that, it's fine. But if you're not, your girlfriend is playing a dishonest game with you. It's the dishonest way she's going about it that worries me.

Maybe you need to do some reading on the subject and decide if that's what you have in mind. If you google domination and submission you'll find plenty of information. Then make your own decision based on what feels right to you.