Fair questions.

IMHO, conservative Christianity states its prescription wrong. It says, "Sex is wrong unless....(archaic rules about dating, monogamy, marriage, heterosexuality, american dream etc)" Whereas I think the approach we should take is "Sex is fundamentally good unless... (lack of consent, abuse, dishonesty etc).

I don't think reason why the Bible might have insisted on virgin-til-married really apply. First (See scholar Michael Coogin on this), the concept of virginity only applied to women. Second virginity was valued because it was a literal currency used, via arranged marriage, to cement alliances or exchange livestock. Obviously we don't use marriage for those reasons anymore, so I think vestigial notions of virginity, and sexual purity are mostly cultural. Third, some passages in the OT look a like more like sex-positive feminism (the book of Ruth and the story of Judah and Tamar) than they do like conservative Christianity's ideal of passive, virgin woman.

Why am I non-monogamous? Well, sex is good. My partners know about each other. They're not monogamous either. It's something I've wanted to explore for years and now I find myself in a position to do it. Neither partner expects more commitment from me than I can give right now (this includes 'pursuing marriage', eventually making babies, and all that other American dream stuff), and vice versa.

I guess I don't feel like I need more reasons other than a simple profound quote from an Chinese philosopher. "That which is crooked in not to be straight. That which is short is not to be lengthened." In other words, I'm intuitively in tune with having my relationships like this, and I don't want external, artificial, pressure to confirm to something that is not natural or intuitive for me.

Naturally I don want to avoid things like abuse, dishonesty, or anything that makes sex bad. Though honestly, does being married and staying a virgin til you get there avoid those better than the alternatives?

So there you go.

tl;dr The conservative Christian sex rules aren't really 'Biblical' in my opinion. I think sex is fundamentally good. I am in poly relationships because that feels right for me right now.