Britney Spears' Boyfriend Sam Asghari Tight-Lipped on Proposing Marriage

Britney Spears' hunk of man is happy to open up about their gym life, and even her music -- but you gotta see what happens when the big "M" comes up..

We got Sam Asghari leaving Bristol Farms Wednesday in Westwood, where our photog first asked who was in better shape between him and his gal. His answer might surprise you, and so might his favorite Brit workout song.

Then it got really interesting -- as Sam slid into his convertible Mercedes, we asked if he plans on proposing anytime soon, and he answered directly. If nothing else, you can tell he's prepared for the question. He should be ... they've been dating for nearly 2 years.

One other thing ... the topic of Britney reportedly agreeing to pay K-Fed thousands more per month in child support -- after a nasty court battle -- is clearly Sam's kryptonite.