Haruka was never this.... bold. This seductive. Oh, she loved to flirt... even while with Michiru, she loved flirting, especially with Usagi and the other girls. But that was playful. Teasing. This was a girl who's heart was shattered, throwing herself out there, to see if she still "had it". To see if it was HER fault that Michiru moved on.
Add the fact that she was JUST tipsy enough to kick her inhibitions to the curb, and Haruka leaned in then, and pressed her lips to Alex's, curious as to the reaction it would get.