Susan’s answer to “Does anyone find it weird that Anna had strong feelings for and slept with Dexter even knowing that…” > Likes and Comments

You can be a strong, independent woman and still be attracted to people and want to have sex with them. Aside from that, Anna clearly has "daddy issues" stemming from her father no longer taking her with him on business and then leaving altogether without a mention or trace. I also think she always felt "bad" about sex in general because of her first relationship and maybe she thought someone like Dexter had an interest in her the way other men didn't. It's really only human nature and hard to judge a character on that.