I can understand the caution around pregnancy with partners outside a marriage. Custody issues are complicated enough within monogamous settings. Even between my wife and I, when we were at a point where we didn't want kids for a few years we considered me getting a vasectomy and freezing sperm, going so far as to set an appointment for the surgery, but ultimately Ginko decided to opt for an IUD for simplicity of reversal.
With people outside the marriage, one girlfriend couldn't get an IUD to work for her and I don't consider condoms to be enough unless I'm ok with pregnancy, so we never had PIV sex. With another girlfriend, we use condoms plus IUD, while I'm considering going back to no PIV with anyone other than Ginko for simplicity. I enjoy non PIV sex more anyhow.
Once Ginko and I have had as many kids as we want I'll likely get a vasectomy. If we ever want more kids afterward there's the possibility of adoption, though that may be more complicated if we're openly poly.