I think a lot of men are threatened by actual lesbians IRL, but are nevertheless turned on by lesbian sex. So I'm going to address your question in terms of what men see in lesbian porn and erotica. Apologies if you find my response to this very frequent question too porn-focused, but I think that's where the appeal actually lies.
On the superficial level, I think it's two main things. Men find women's bodies incredibly attractive, and seeing two together doubles the visual pleasure. Plus, seeing two women together may be less threatening than male-female scenes, by removing the competitive/comparative aspect of having another man in the picture, who of course is almost always hotter than the viewer.
A little deeper, I think it makes for an action free of the traditionallly coded-and-loaded baggage of male-female interactions - much as gay-male porn often does for women and especially lesbians. Of course, that's despite the fact that lesbian porn is one of the least realistic genres (or so lesbians tell us), because it's primarily produced to satisfy the straight male gaze, while gay-male porn is seen as one of the most realistic, because it isn't.
Another reason for me personally is that as a very strongly bisexual male, I'm heavily turned on by same-sex sex, regardless of whether it's men fucking men or women fucking women.
Finally, while I enjoy lesbian porn all of the reasons stated above, I like that it usually at least takes a shot at portraying the scenario, with tension and uncertainty, leading into a same-sex encounter, in a way that I have always wished bi male porn would do - and rarely does. I find that there is hardly anything more emotionally charged, tentative, delicate and ultimately hot than the prospect of two straight-presenting men getting together (with or without a woman present). And bi male porn gets it wrong every fucking time. Seriously, I probably wouldn't have published a hundred-plus bi male stories on Lush if bi male porn weren't so awful. So despite the often abysmal acting and supposed "unrealism" of how it portrays actual lesbian sex (not like we guys can tell), when I see two women working through across that boundary and doing these sexy things together, my mind sometimes drifts to imagining a similar interaction between myself and another man, giving me what bi male porn doesn't. I know of at least one or two other bi Lushies who experience this too.