Dear 100 Hour Board,
Nebula and Tony Stark must've been on that spaceship together for a pretty a long time. Do you think they ever...you know...
Smelled each other's farts? Yep. I think they did.
The Wikipedia page for Nebula claims that, in accordance with the Marvel Cinematic Universe, "Every time she lost a sparring match, Thanos subjected Nebula to torturous mutilation, replacing parts of her body with cybernetic enhancements for further improvement." It also cites that "Nebula is an athletic woman, and an excellent armed and unarmed combatant."
Based on these facts, I would guess that Thanos replaced her...you know..."down there" with some cybernetic enhancements because that area of a woman is particularly sensitive and therefore a potential liability in physical combat.
With that in mind, and taking into account Tony Stark's marital status, I'm willing to bet that no, they never "did the deed."