Fitz is obvious, but Simmons has never shown any romantic sentiment toward him. It's not like he hasn't been willing to die for her out of love before - and last time he got a hug and an "ehhh sorry, not feeling it" in response.
I don't think the guy's commitment and strength of character has ever been a barrier. That was clear by the end of season one. "Do heroic things until they fall in love with you" is TV logic, not actual-human-relationship logic - which is why I'm disappointedly expecting they'll take it in that direction.