Grandson could use advice, not judgement
Q: My 17-year-old grandson bought his 17-year-old girlfriend of barely two months lingerie from Victoria's Secret. I think this is very inappropriate. He thinks I'm a dinosaur. Guidance, please?
A: I think if you were his parent, then this would be a great "last call" bell, telling you that you're about to have little to zero say in your son's sex life. If you're a grandparent but acting as his guardian, then that applies.
If you're a grandparent in a traditional grandparent role, then you've got very little to say here except as an academic exercise.
Unless, that is, you're able to communicate with him — as guardian or grand — not the judgmental aspect of your thinking, but the substance behind it.
So, instead of shooting him down as "inappropriate," which just begs him to get defensive without providing much enlightenment, try explaining what you believe. Do it in as accessible a way as you can: "It's your business what you buy with your own money, of course. Be careful about moving fast with new people, though. It takes a long time before you really know someone, and when you fall hard it's tempting to get serious right away." Or similar. Stick to the theme that gifts can speak for us in ways we don't intend.
This is, again, assuming you're able to communicate this way with him. It's another reminder of why it's so helpful to establish early with kids that they can talk to you about difficult things without your freaking out on them. That buys you a lot of leeway when they're older and you have an I'm-Older-So-I-Know-type opinion you think it's important to convey.
Would I use one of those precious opportunities on this specific issue? No, not unless there were context to support that his moving fast was a pattern.
All this being said: If you just think it's too sexy for 17, then, probably best to see this as a stolen-horse/barn-locking-type situation.