Q. Is your g/f already poly? A single person with a monogamous background might have different expectations of your relationship than someone who's used to being poly and sharing. Your marriage does come first and you need time to achieve a balance that works for everyone.
I have a husband and kids, so does my boyfriend. We have never had a sleepover...and it's okay. Though I'd like to have one, it's difficult to plan with two households to consider. I think that in time, it's important that your wife meet your g/f. Not saying you should push hard, but your g/f unwillingness to meet could signal some jealousy issues. I also found that I wanted to meet my hubby's love interest because it makes me feel closer to him and he knows mine. We all get along like good friends and occasionally hang out together for movies and games nights. It makes us 'family', it's a good feeling.