I am going to throw my 10 pence worth in, as someone who writes SF with a romantic subplot. I think part of the problem you've got here is that what you want to write is not romance, so looking at Hallmark and romance novels is not going to help you. What you're asking is how to write believable female characters and believable social interaction, so relationship stories.
I would suggest having a look at some feminist, female gaze scifi - left hand of darkness, woman on the edge of time, the female man etc for hardcore female POV. I would then have a look at some of the more character driven scifi women have written, so as has already been suggested, vorkosigan saga, perhaps Emma Newman, Octavia Butler. It would then also be worth looking outside SF at some of the big hitting women's fiction writers - authors like Marian Keyes, Maeve Binchy, Jill Mansell, Alice Hoffman, who write a lot about women living through different stages in their lives for a female perspective on those.
Then when you start writing, you need to make sure that the development of the relationship ties in to your main plot arc and the character development. It should fit in with the overall theme of the story, so that it drives the character forward just as much as the SF elements of the story do.