I think most people will have a desire to monopolize attention (esp from a lover). I mean, look at children, how they'll often demand attention, and how a single child will often react negatively to a sibling joining the family and taking attention. But as we grow up, we (hopefully) learn to, uh, control ourselves???? And respect other people's autonomy???? So it doesn't really bother me that Alphonse has those kinds of thoughts, as long as he doesn't act on them. It'd be nice if his reasoning was "no, that wouldn't be right" instead of "no, then she'd be sad and I couldn't see her smile", but he's still putting her happiness over his ego which is good enough for a fictional ML
Also, am I the only one lowkey hoping it's the prince at the door to inject a bit of drama into the plot? He volunteers to act as the male dance partner, Alphonse comes by and sees, his possessive tendencies flare up again............and then we get to see him control himself in the face of a blatant "threat" to his "possession" of Chloe