Rolling my eyes at all the Cool Girl, Not Like Other Wives responses, from women who are just so much more kind and understanding than their "hysterical" counterparts who see red flags in this.
Anyway, I'll break down some of OP's responses, as someone in agreement with other posters that it's AGP. It may not ever progress to the full transgender reveal, but it's clearly a fetish and he's clearly lying about it, and you have the right to have got the massive dose of ick and uncertainty that you did, OP.
So here are some things that jumped out at me.
He told me he's been wearing them for a while. I think he said a couple of years now.
You say you've been together six years, moved in two years ago. It's likely this began then, and it began with him trying on your clothes, when you were out of the house and he had alone time with your wardrobe.
Any items of clothing mysteriously go missing? Become stretched out in odd places? Anything he helpfully took out of the wardrobe and washed for you, that didn't actually need it?
Mostly buying them around Christmas or Valentines because it's easier to get the sort he likes from Asda or Sainsbury's. Said he can see how wide the gusset is and what support they'd offer which you can't really do online.
So, the fetish has escalated to the thrill of buying them in person. I've worked retail. We all knew the creepy guys who were obviously buying for themselves. If he really wanted discretion, he'd buy online. Read reviews, or take the chance on something not fitting. Visiting lingerie departments, at the time of year when the offerings are most likely to be sexy and appealing to his fetish, is a sign of the fetish escalating.
As I mentioned upthread, this whole idea of him wearing women's underwear for "comfort" is bollocks anyway. There are plenty of styles of men's underwear designed for a more snug fit. Briefs exist. Women's underwear is not designed to accommodate a dick and balls. It is not actually going to be more comfortable for him than something he could find in a men's outlet online without all this secrecy and potential shame. Please wake up. He's spinning you a line.
This is also why he has "more than he realistically needs". It's because buying them is part of the fetish. He gets a creepy little kick out of seeing whether the sales assistant will realise what he's doing, and have to hide her discomfort. Fooling her is fun, and making her uncomfortable is fun. The experience is a win-win for him no matter how it goes, and it's a compulsion. He does it when he needs the thrill, not the underwear.
He said it's mostly about how they feel and how they make him feel a bit sexy in himself.
And there we have it. He's openly admitting the fetish element here. He's just trying to soften the blow to you by padding it out with all this guff about the "comfort," and "compression".
Wearing women's underwear arouses him. There is no way he hasn't thought about having sex while wearing them. He likely masturbates in them - which is why he's only wearing them "1-2 times a week" and why they're always freshly and secretly washed. And again, he is lying to you about this. So how can you be sure how far the fetish goes?
Can you be sure you're not being played with here? That this "accidental" discovery you made wasn't planned so he could gauge your reaction? That it's not going to escalate, once you've established how open-minded and accepting you are? Can you be sure "functional" underwear won't escalate to lingerie? That "feeling sexy" won't lead to wearing them in the bedroom, now that you've done what he hoped and broached the possibility? What if the knickers start being paired with a bra, "only in the bedroom"? Because, as many women have attested on this thread, there's a real possibility that's where this is going. There's a real possibility he's testing the waters with you. Have a serious think about where your personal comfort level is, and how you might respond if he springs any of that on you.
You're not being a bigot or a prude if the thought makes you uncomfortable. You have every right to your boundaries.
I guess that makes sense with the styles that were in his drawer - none if them are skimpy and he's not exactly huge down there.
He may want extra room down there, to make other activities easier. No fun cutting off the blood supply, is it? Sorry.
His particular fetish may even be to look like a "normal, everyday woman" - or even like you. Just because they're not lacy scanties doesn't mean it's not a fetish. Again, sorry.
There's also the possibility he planted them where you would find as a way to open this conversation between you. In which case, he's starting with the soft option that is least likely to scare you, and is working up to a reveal of his true tastes. Again, I'm sorry. But all of these are real possibilities.
He doesn't want to wear other female clothes because people would think it's weird, and he wants this to stay private, just between us. He also confirmed he doesn't see himself as a woman and has no intention of going out dressed as one.
I notice he hasn't ruled out wearing other female clothes in private, with you. As part of his fetish. He only wants to keep it behind closed doors.
He has lied to you at least twice already about this, and as others have said, this tends to be a fetish that escalates. I'd be very cautious abou what he's promising you here. Remember, he's already taking this out in public by choosing to buy his underwear in-store. There is no good reason for that. The ones he's given you are absolute bullshit. He's doing that because it's a little thrill and he gets off on it. Do you really think he won't want a bigger thrill at some stage?
Final note: the scenario you give of how this all started? Him "forgetting to pack enough" underwear on a trip away, so he had to borrow yours? That was likely a lie. He likely set the whole thing up, so he could manufacture a situation just like the one you're in now. It all reeks of manipulation.
I know you think he's a great guy, but I think he's playing you like a fiddle. Please don't fall for it. Please be wary. You deserve better than this.