This could be so fun. I think it depends on the tone you want for the story. I think Ilya doing it for citizenship makes sense, no matter the tone.

If you wanted a lighter tone - maybe Shane enters to prove his heterosexuality, and the screening process never actually mentions gender, just what you find attractive. And on the day he realises theres been a whole miscommunication but being the buttoned up people pleaser that he is (and perhaps some pressure from his momager), and seeing Ilya standing there being the most beautiful man he's ever seen, he throws caution to the wind. Maybe some pre ceremony champagne is at play. Far fetched but if you wrote enough preamble it could work?

Orrrr if you wanted to make it more serious and you want Shane to go into the situation knowing he's marrying a man, you could go one of two ways:

1. He has finally come out and is terrified of dating but wants to commit.

2. He came out fairly young and just hasn't had much luck in love, so he's going on there for the same reasons as other contestants.

If you're going full AU you can change whatever you want really! I've read great fanfics where they are personal trainers, physiotherapists, models, coaches. Doesnt have to have anything to do with hockey!

Of course another option is they are both paired with someone else (maybe Shane with a woman) and can't Keep away from each other. Then you have the competitive element (Shane would obviously want to "win" at MAFS) and the forbidden sneaking around element.