He never forced her to marry him... he just gave her an offer he knew she wouldn't refuse.
His motives are.... confusing at best.
Quite frankly, he probably had quite a rough childhood considering his upbringing and the racism that followed with it.
Followed by his mother dying I'd assume he'd be super shaken up.
This caused his motives which Auron describes as the spiral of death.
Really, I can't make much sense of his motives myself... I just love him because fighting him is a blast and his theme is amazing.
Final Fantasy X Soundtrack: Seymour Battle Theme - YouTube
EDIT:
Yes, I know I just made a complete 180 from my earlier post. I no longer agree with what I said earlier, shut up.