Because it's my headcanon that Robert really, really would like to have a threesome with Ned and Catelyn because firstly, he's Robert, and also because he seems to basically see Ned as his soulmate/the one who got away, and also Catelyn is a pretty lady with pretty hair and he still likes pretty ladies a lot, too.

SO, I would love Robert to suggest it to Ned, HOW NED REACTS IS UP TO YOU TOTALLY. XD