I feel like not everything has to be directly stated. Obviously we all know she is a lesbian, even hussie has refered to her as such multiple times. But it's also part of her theming within the series. The fact Karkats keeps insisting that her and John HAVE to get married and HAVE to have babies is obviously hitting at misogyny and heteronormativivity and the fact neither of them actually wanted that(and Rose never even entertains the thought) says a lot. The way that the only possible heterosexual parings "hinted" are all her being forced by men she is clearly not interested in (Doc Scratch, Eridan, John bc of Karkat) and when she rejected Eridan his immediate fucking reaction was "She is soooo into me". And Doc Scratch is a groomer so. Basically, all possible romantic/sexual relationships with men are her being forced by others or sburb into it, all while she ignores or outright reject those and instead goes for the Lesbian Alien Vampire Lady. I don't think it can get more obvious than this