Basically monogamous, but I can imagine having a triangle relationship (tried that a few years ago, didn't go well, but if it does, why not). I know that it's possible to love more than one person. Also, I think it's OK to have sex with other people, when the partner agrees (of course, you'll have to be honest and talk about STDs and safety).
I find it strange that "relationship" automatically means "relationship between two people", and everything else is out of scope, impossible, immoral, without legal framework.