Depends on what you mean by “sexism”, I think.
I’d say that some homophobia reflects the sexist idea that women are inferior to men and that men who behave in an effeminate manner, or women who refuse to “submit” to men sexually, are loathsome perverts. I don’t think this can explain all homophobia, though.
Sexism could also be defined as a belief in rigid gender roles, and that there is something wrong with people who do not adhere to these roles. I don’t mean simply observing that men and women in general sometimes behave in different ways, but believing that a woman should not or by virtue of being female can not work as a mechanic, manage her own finances, vote, etc. Or believing that a “real man” wouldn’t be willing to change his baby’s diapers.
A person could possibly believe in rigid gender roles without believing that the feminine role was less important than the masculine one. But it’s still sexist to believe that people of one sex should not do certain things that are perfectly acceptable for people of the other sex. For instance, it’s sexist to believe that promiscuous men are admirable studs while promiscuous women are the worst kind of trash. I don’t think it’s a stretch to say that it’s also sexist to believe that it’s fine for a man to have sex with women, but not for a woman to have sex with women.
Homophobia is a form of believing that all men and only men should behave in one way, while all women and only women should behave in another. If that’s not sexist, I don’t know what is.