Marrying your partner seems to be quite common - is it really?
Yes, it’s very common. You’re working full time, training, problem solving, competing with one person and you inevitably develop a pretty deep bond (or sadly in some cases end up hating them haha!). Many long term partners end up married, while some are platonic “soulmates” for want of a better term. Unfortunately partnerships do also end badly sometimes.
How cutthroat are the competitors?
Not as bad as you might expect in most cases. There are rivalries and couples do get annoyed when they get beaten by their rivals, or overtaken by couples that have historically done worse than them, but the vast majority would not take it out on the rival couple and would be amicable to them when seeing them outside the competition in lessons etc. Usually the worst you get is some dirty looks, although I have been at comps where name calling and shouting has happened. There are also well known stories of people trying to “sabotage” others though in some pretty elaborate ways but that’s extremely rare!
Most of your close friends also tend to be other contemporary competitors.
Probably in general the most cutthroat aspect is when one partner in the couple has secret try outs with another dancer - that’s pretty common!