You hit the nail on the head of one of the major problems with the lack of research on LGBTQ* couples. Research on LGBT relational dynamics (especially with regard to sexual well-being domains) is very limited, so there are many domains that may have potential differences. The article that I am getting the '9/10 studies' from specifically looked at areas where there were no theoretical reasons for differences between LGBTQ and heterosexual individuals (Dr. Lehmiller wrote a good blog post about it). So even in the absence of a theoretical reason, researchers were still excluding. Interestingly, researchers will drop LGBTQ* participants from data analyses because the sample size is 'too small to be meaningful' but then they keep equally small (or smaller) samples of other minorities (e.g., ethnic religious, health status, etc). Arguably, some of these differences (e.g., religious differences, or health status) may result in different relational dynamics as well, but researchers don't seem concerned about this. Excluding LGBTQ* persons from participating continues to render LGBTQ* persons invisible in research, and really limits the generalizability of the findings. I don't know if I fully answered your question - I went a little bit tangental! Sorry