Pepper Potts was originally Tony Stark's personal assistant, but later went on to be the Chief Executive Officer of Stark Enterprises. Talk about sleeping your way to the top. I'm just kidding. The only reason Tony Stark even became attracted to her is because she's incredibly capable. Hell, Tony Stark can get any bimbo he wants, but if you're going to be marriage material, you better be extra special.
The messed up thing is that the only reason Tony Stark proposed to her in the movie is because Spiderman decided he didn't want to announce he'd be joining the Avengers. Tony Stark had to scramble a plan together since he'd put together a big press conference for the announcement. What does he do? Proposes. It's kind of genius, but it only worked because Pepper Potts is awesome.