1) Literally, two or more otters physically fighting during mating season for a mate or breeding territory.
2) When two people start to have sex but end up fighting each other in an otter-like fashion. It's cute but surprisingly painful and usually both parties involved have severe injuries.
I invited my boy over for some late-night lovin', but it evolved into an otter fight, and we woke up all my roommates and broke a lamp. Luckily, my roommates are freaky and like to otter fight too because they joined in.