I don't consider it any less believable, frankly, than William Shatner's supposed animal magnetism, that his Kirk can get ALL the women.
Maybe because you're looking at Shatner through male eyes? TOS era Kirk was a very handsome man, and personally I feel that Shatner was at his best in terms of appearance in the movie TMP, he looked very fit, and he had added maturity over '60's Kirk.
As you wish ...
Besides, he didn't get ALL the women... Gillian Taylor didn't give him the 23rd-century equivalent of her phone number, or even the name of the ship she was going to be serving on. All he did get from her was a kiss on the cheek and "
I'll find
you."
No wonder she rejected him. By then, of course, Shatner had bloated out, considerably. His nose had gotten bigger and redder. And his
STAR TREK IV: The Voyage Home Hair Piece had the tightest curls out of all of them, making his fat head resemble Bilbo Baggins from
THE HOBBIT cartoon, that came out in the olden days. I loved it how, in
STAR TREK V: The Final Frontier, Kirk goes, "when they put
me out to pasture, I hope I fair better than Kor." But no-one had the balls to say, "Well, Kirk you
are on the pasture, but the
grazing isn't good ..." He was an old man in TVH. He wasn't going to get Gillian. Hell, he probably doesn't even feel sexual urges, anymore, without the help of
Viagra.
Meanwhile, back on topic ...
I agree that her character was written rather poorly in the beginning, but she didn't even give it an entire year before she bailed on the show.
One needs to look at what they ended up doing to pretty much all of the characters on the show to know that Tasha Yar would've been developed beyond what she had to work with in the first (half) year.
Yar started out being relevant and quickly hit the sidelines. Denise was young and beautiful, and as far as she was concerned, Leading Lady material. But out on her own, was she going to be a draw? That was the Great Unknown and the gamble she eventually took. Unfortunately ... sheesh ... even after taking her clothes off and just throwing her raw sex at the screen and in magazines ... nobody cared. The kicker was, she hadn't burned her bridges with TNG, entirely, despite the insult of her character being killed off by a trashbag in an oil slick.