Well, that was just a reflection of the fact that female viewers of the show were fascinated sexually by Spock. He was a huge sex symbol at the time, and this got a lot of attention in the media, probably more than any other aspect of the show. So I don't think Blish was projecting any sexist assumptions onto things; he was just reflecting the reality he perceived, namely that Spock was fascinating to women.
Plus it seems entirely plausible that Kirk is the sort who would pay attention to what the ladies around him respond to to an obsessive degree -- even if this were a sexist attitude, well, some people have
sexist attitudes, and Kirk doesn't have to be past reproach. Having sexist characters doesn't necessarily make a story
sexist, it's more complicated than that. (But what's sexist about portraying women as taking a sexual interest anyway? Men certainly swoon over alien babes in Star Trek
all the time.)
It's why Undiscovered Country
may be my favorite Kirk story, and I think one of the more daring stories Star Trek
has ever told - it showed the star of the show being openly racist. That's obviously not worthy of celebration in itself, but it forces dealing with the issue in a major way (vs. just encountering a racist villain), and it's nice when a franchise is bold enough to challenge it's audience by requiring them to reevaluate their hero. It also makes it more relevant to have a story about how we might get from today's A to the B of Star Trek's purportedly net-average more enlightened humanity.
And in the context of Trek lit, that's why I found the intense audience reaction to Sisko's behavior in DRGIII's Typhon Pact
novels very interesting. Because people didn't just complain that Sisko was acting out of character, they also complained that it wasn't OK to portray a leading man acting that way, and specifically a black father at that.