"...knowing that bringing up children is one of life's hardest jobs ever and wanting to be really really bloody picky about who i am willing to share that job with."
I actually think this is the most important thing, and I think the expectation of heteromonogamy has positives and negatives to this. Firstly yes, if you're aware of the influence of your partner on any potential children and you have a good relationship template to start off with and you find someone really good who you really think would be great at it, then heteromonogamy is great in that situation, because it is expected that you'll stay in this arrangement until the children have grown up and left home, and even if you do split up, society expects the father to continue to be involved in his children's lives. But if you have the opposite - an unplanned pregnancy, or a bad relationship, or pressured into it, and you make the mistake of putting the father on the birth certificate, there is no way you can undo this mistake and get him out of your child's life, if they're a bad influence or manipulative or whatever, the best you can hope for is that they disappear.
I think children do need positive role models of different genders, examples of positive relationships (which don't have to be heteronormative ones, just respectful ones) but I DON'T think this needs to be within the family home. I just feel that I was let down by my childhood examples of male behaviour and what relationships were like, and while I was totally happy to raise DS as a single mother (As my mum did, and she was brilliant) I just completely disregarded the importance of his father and assumed he wouldn't make any kind of influence on him, or something. And I think that was a mistake. If DS' father wasn't his father, if he was an uncle or something, I wouldn't let him see him, and that strikes me as ludicrous that just because he happens to be a parent that there's this expectation that they should have some kind of relationship, when XP has never really bothered to be involved with him.