Mages aren't actually FORBIDDEN to marry, as it happens.
There's this banter:
Alistair: So tell me, you have any children? Grandchildren? I don't know, great grandchildren?
Wynne: What would make you think I have any children at all? You have to know I've spent most of my life in the Circle of Magi.
Alistair: You just seem like the grandmotherly type to me, I don't know.
Wynne: I suppose I'll take that as a comment on my demeanor and not my age.
Alistair: Mages aren't forbidden to marry or anything, are they? It's not such an outlandish question.
Wynne: Isn't it? What sort of man would marry a mage, do you think?
Alistair: How about another mage? There are just as many men as there are women within the Circle, as I recall.
Wynne: That sort of union is... not encouraged. Although that does not stop us from seeking out each other's... company from time to time.
Alistair: I... all right, suddenly you don't seem quite so grandmotherly to me anymore.
Wynne: Good. I would hope not.
And this conversation female mages can have with Teagan:
Teagan: If I may be so bold, what of you, my lady? Are you married?
PC: I'm a mage.
Teagan: Mages can marry, or so I'm told.
PC: They can, indeed, if the man is brave enough.
Teagan: I can think of several reasons why one would be willing to be so brave.