"Do many straight males wear thongs? If so, how do I convince my wife that they are sexy?"
I can tell you that I get this question a lot and I’ve been lucky enough to speak to both sides about it (husband and wife).
A word of caution: don’t try to “convince” her that they’re sexy (i.e. telling her that a lot of straight guys wear thongs). From women I’ve spoken to it ends up coming off as an attack/challenge: ‘Their wives/girlfriends like it, why don’t you?’ It’s going to put her on the defensive and can make her feel inadequate, inferior or even incompatible.
The best way that I’ve heard this conversation handled is actually by taking a step back and broadening the conversation so it’s not just about one specific thing that you want. At the most basic level, this is a conversation about attraction and sex. Open and honest conversations about attraction/sex are really difficult, filled with pitfalls and therefore tend not to happen as often as they should. Ultimately, you want to mutually feel that you can talk to the other person about anything so I would encourage you to have a broader conversation and frame this as just one of the discussion points.
When it comes around to thongs, own it. Tell her that wearing thongs makes you feel sexy and add a ‘because’ (i.e. “because I think my butt is one of my best assets and a thong is the only underwear that doesn’t cover it up) to help her understand. Ask her how she feels about it and be careful not to be judgmental of what she says; remember that you can’t argue with the way she feels and you want her to feel comfortable sharing. If she isn’t amenable to the idea, find out why. Maybe it’s a misunderstanding that can be easily remedied. I won’t pretend to try to address deeper issues here; there are a lot of very talented people who make moderating these discussions their profession!
"Q: I wore a pair of my wife’s black briefs by mistake one day. Kept them on. They were very soft and comfy. Do they make similar nylon/spandex briefs for guys?"
A: The challenge is that everything will either be much lower rise than the pair you tried or have a waistband/fly. I think this Naked Brief is closest to your description.
A lot of guys actually find women’s underwear more comfortable (and as you can see there’s not a lot of stuff made of similar fabrics for guys). Some of the product reviews for women’s panties are even left by guys. I’d recommend something that isn’t too low rise so that there’s enough room like this Bali Microfiber Hipster.