I honestly can't tell whether this comes over as defensive, and if it does I apologise. I am a fan of all the m/m pairings you list (though Spock/Uhura is pretty awesome as well).
I can definitely see the pattern you are describing - but I'm not sure whether it's actually about the female character "being good enough" for the lead.
After all, in all of the pairings you mention, with the possible exception of Merlin/Arthur, who is the fans' favourite character? It's not the lead, but the secondary male - Daniel, McKay, Spock. To me, it seems far more like fans wanting to build on and expand their favourite characters' relationship with the lead. It's not about the woman being "good enough" it's about her forcing out your favourite.
I can't answer for all fans of the pairings, but I've seen this borne out in the way that fans are far more supportive of other pairings that involve their favourite - I know a lot of very anti-Sam/Jack J/D fans who are happy to read Sam/Daniel, and I don't think I've ever seen a negative comment about Rodney/Teyla (though that may be the circles I moved in).
From metafandom
I can definitely see the pattern you are describing - but I'm not sure whether it's actually about the female character "being good enough" for the lead.
After all, in all of the pairings you mention, with the possible exception of Merlin/Arthur, who is the fans' favourite character? It's not the lead, but the secondary male - Daniel, McKay, Spock. To me, it seems far more like fans wanting to build on and expand their favourite characters' relationship with the lead. It's not about the woman being "good enough" it's about her forcing out your favourite.
I can't answer for all fans of the pairings, but I've seen this borne out in the way that fans are far more supportive of other pairings that involve their favourite - I know a lot of very anti-Sam/Jack J/D fans who are happy to read Sam/Daniel, and I don't think I've ever seen a negative comment about Rodney/Teyla (though that may be the circles I moved in).