Meg is only seen, being attracted to men, but given the fact that she is actually a man trapped in a woman's body, she plans on being a man later on, meaning that her overall sexual orientation would be homosexual.
In the episode "Brian Sings and Swings", Meg joined an after-school club, feeling as though she'll finally be included in something. It turned out this club was a Lesbian Allience club. Meg still wanted to be in the club anyway, so she started identifying as a lesbian. Lois told her that she wasn't gay, but Meg ignored her and stayed in the club and started a relationship with a girl named Sarah. She tried being a lesbian, but it just didn't work out for her because she didn't want to do anything with Sarah. She tried kissing her, but only gave her a peck, and when Sarah wanted to have sex with her, she refused and admitted she wasn't interested in women.
In the episode "Stu and Stewie's Excellent Adventure", Stewie travelled into the future, where the kids were all adults. Meg had been given a sex change surgery and was now a male named "Ron Griffin". He took great pride in the fact that he was male and was always talking about it, much to the annoyance of the rest of his family. He was not actually seen expressing any forms of sexual attraction for anyone else in this episode, but it can be implied that he still liked men just as much as he did, back when he was still a girl.
In the episode "Dial Meg for Murderer", She was seen passionately making out with Connie Damico. At this point, she had become more comfortable with making out with women than she was before, but this doesn't mean that she was neccesarily sexually attracted to them.
In the episode "Quagmire's Dad", Meg complimented Ida on her appearance, after she had gone through a sex change surgery. Ida responded by asking her who did her surgery.
In the episode "New Kidney in Town", Meg believed that homosexuality is a choice.