I prefer to date married men. I am married myself, and I only want secondary relationships where we see each other now and then. By sticking to men in solid, healthy poly marriages, I can try to avoid starting a relationship with a guy who needs more than I can give him.
A married couple will, or should, always have less time for outside relationships than for their own. They are the priority. It sounds like those two may not have a very well established agreement as to how many dates per week are okay, or so forth, which may be a yellow flag.
As someone from the outside dating one of them, you have to accept the fact that their family life comes first, and that you and her enrich one another's life - but she cannot complete yours or give you everything you need. It sucks not to be able to spend as much time with someone we love as we'd like to, but we have to accept what our poly partners are able and willing to provide. If that's not enough, then you could seek out other people to date to satisfy your romantic/social needs.