yoga pants are good. but underwear that even has the capability to show panty lines, no good. you should be thonging it regardless of the pants you're wearing, if you're concerned about what guys think. most guys I know like thongs. a few like the boy shorts, but that's too much material for me. but as a general rule, the less fabric, the better. full-backed underwear is no dice in any situation. when my girlfriend is wearing a sundress, or some other light fabric, and I hug her and can feel the outline of her thong outside her clothes...makes my pants shorter every time, lol. sorry to go off on that tangent, but it's a major part of my opinion. but yoga pants on their own usually look hot, to answer your direct question.
Oh my god when I see a women in yoga pants I almost start drooling.No lie and panty line showing I turn mentally challenged at that point.I think they are sexy as hell.that is why I don't do yoga I would seriously hurt myself in the room from looking around.lol
Yoga pants are good, but girls should know when/how to wear them. And if panty lines are obvious enough to notice that's often times trashy...but if a guy goes out of his way to look and can notice a subtle panty line then that's totally different.