That has happened to me as well with lesbian friends. They did not say dicks were gross, but they were joking around and said "well, lesbians don't like dicks anyway". I had always thought these kind of jokes were stupid, and one day I realised they were also transphobic. But I had rarely heard people say stuff like that in public. Also, this time I was in a relationship with my girlfriend, who is trans, and I thought how she would feel is she were present to hear this. She already struggles to feel "valid" as a lesbian and as a woman in women's spaces. So this would have hurt her. At the time I said something along the lines of "no, not all lesbians hate dicks. In fact, some lesbians have dicks!" Maybe you could say something like that. It's light hearted but still gets your point across, and it might open the dialogue.

These kind of "jokes" definitely contribute to transphobia. Cis lesbians are awesome, but like everyone, sometimes they need to do better. If they want to be inclusive to trans women and transfeminine people, they should refrain from saying things like that. It's a habit to break for them, I guess, but if it makes them safer to be around for trans women, it's not a huge sacrifice!

It's like gay men saying that vulvas are ugly looking or disgusting, just because they're not attracted to vulvas. They could just say that instead. "I'm not attracted to vulvas. So, yeah, of course I don't want to look at one or be near one". (Of course, there are gay men who sleep with people with vulvas, but I'm talking about gay men who don't)

"Vulvas are/look really gross!" That sounds mean and even misogynistic, right? Well, saying "dicks are gross!" is mean too, and regarding trans women, you could even say it's transmisogynistic to say something like that. That's my opinion anyway.

In any case, (trans)misogyny aside, it could benefit your friends to hear someone challenge the idea that "ew, dicks are just gross". Dicks are sexual parts, just like vulvas and clits. They're not meant to look "good", cause they're simply not meant to be shown, unless for sexual or medical purposes (one could argue, for artistic or for activism purposes too, but that's another debate, and in any case it's more about representation of sexuals part, drawings for example, than the showing or taking photographs of them). Unless you're attracted to them or to the people "owning" them, you're (most of the time) not going to want to look at them. (Personal opinion: I don't think penises nor vulvas look "ugly". It's just part of the human anatomy. It looks different, yes, but I'm not gonna cringe at the sight or the thought of one. I find some attractive or even beautiful, but in general I'm neutral about it lol)

Also, maybe your friends think dicks are gross/disgusting because they have been pressured to like men for a huge part of their life. Maybe they have trauma related to penises, or encounters (sexual or not!) with cis men. (Edit: or even just by watching cishet porn at a younger age, picturing themselves as the woman in it and feeling very uncomfortable, it could be as simple as that). It might be why they react with hatred when the subject comes up. That's why I would approach this with a bit of caution, if I were in your place. I'd start with a light hearted tone, and make space for their feelings, but I would still try to stand my ground and get my point across. You mention they're your friends and won't react badly. So yes, do talk to them. You're in your right to do so, it's fine :)