Heat in the Night — Margaret L. Carter

Vampires don't fall in love with each other. When a female enters one of her rare heat periods, she chooses a mate for a night or two of raw sex to satisfy their mutual lust and possibly conceive a child. But young vampire Ingrid, brought up by human foster parents, has absorbed human notions about love and romance.

When she begins her first estrus, she gets thrown together with her dear friend Bron, who can't stand to let any other vampire have her first. Realizing she has fallen in love with Bron, Ingrid embarks on a night of wild passion, determined to keep him from learning her shameful secret -- that one night will leave her with a yearning for love that would baffle and repel most of her kind.