"Maqué" comes from the word "maquereau" which means "pimp". So, at first glance the expression “je suis maqué(e)” seems vulgar.

However, the Larousse dictionnary says that "se maquer" means "to live in a household with someone" and is considered as "très familier" and not as "vulgaire".

So, two things to remember:

1. "Je suis maqué" means the contrary of “je suis célibataire”.
2. "Se maquer" isn’t a vulgar, be cautious though, since it should be used in informal contexts.

N.B. I can’t help you further, since I am not French nor living in France.

"Être maqué" is just an informal way (close to vulgar) to mean someone is in a relationship (often married, but not necessary). It’s not used so often.

And yes, I can totally say "Je suis maqué" on a friends talk, but never on a professional context for example. I can totally say that a friend "est maqué", it’s not insulting, just really informal.

Moreover, "Seul" means "Alone", so in your context, the contrary of "Maqué"