He even took the time to admire the back of it, her ass practically hanging out there for him, and what did he do? Turn her down. There was definitely no expectation that she'd return after the initial storming out, but there she was, slamming a specimen cup down in front of him and all he could do was look at her, puzzled. Find someone else? The hell she was. He already felt like a failure. Seeing her knocked up with someone else's baby was the final straw there.
Tony picked up the cup and stared at it for a good minute or so before he clenched it in his hand and went upstairs after her, asking Jarvis along the way where had she gone in the house.