SPOILERS SPOILER

Shipping kind of falls under "They are sexy, therefore they should have sex."
How much justification you need for them to have sex depends on the individual.

Maybe it's just me, but I got the impression that Loki was somehow influencing people on the Helicarrier through his staff.  We had two different shots of Banner and Stark (I believe) rubbing their temples and Banner unconsciously grabs Loki's staff.  We also had Natasha remain unmoved by Loki throwing her past in her…