It depends on the venue of the first date, usually it would be out for a drink or meal. I would go for smart casual, nothing too revelaing as it gives out the wrong signals, (unless that's your aim, then they'd be the right signals)
If I was hoping things would progress beyond first base, then my underwear would probably be lacy, black with the bra matching the pants. No thong for me.
Sounds like you are expecting more than a quick drink 857Dnegel? Don't wear posing pouch or a thong, they are a major turn off for most women. If you have a good body, then jersey, snug fitting boxer shorts are nice, otherwise a plain, loose fitting cotton pair.