I think I do read less romance into the equation when there's more people involved, just because I can't see all the characters ending up together (3(+)somes being an exception). In The Pure and Simple Truth I feel the ending heavily implies that Draco and Harry are going outside to kiss and settle into a relationship, but in Responsible Science I feel like Bruce could eventually end up with any of his interests and have a Ron/Pansy type friendship with the others. The Bruce/Natasha, Bruce/Steve, Bruce/Tony, even Bruce/Pepper all sort of combine into a Schrodinger's pairing with Bruce in some sort of gen-limbo.

I think who the people are matter too--admiring a friend or relative is different than admiring a(n ex)mortal enemy, with people more likely to read sex into animosities than friendships.

But I'm also inclined to think cannon relationships are gen (i.e. the Avengers (if it were a fic) would be gen even with the wonderful Pepper/Tony scene because Pepper/Tony was established in the Iron Man movies) as long as they aren't the focus of the story, so you might want to take my opinions with a pinch of salt ;)

I think either the & or / work fine for Responsible Science, but I think the & makes more sense in your case because the fics aren't explicitly shippy and the attractions shouldn't squick anyone (as opposed to Sam/Dean incest or Loki/horse whateverness). I think A Fine Spur is less gen than the others just because the Natasha relationship is the only one. The thoughts being explicitly sexual doesn't matter so much, I don't think, because I feel like Bruce is more restrained with Tony or Steve since Tony is in a relationship and Steve may or may not have a blind spot, while Natasha owns/understands her sexuality?

I've had way too much fun reading the comments here and on lj!