finally, a rational mind...
Yes, cheating is cheating, but by 'literal' definition, a female can't 'cuck' you...she ain't male and can't get your woman pregnant.
I try not to get into the mind of those who don't enjoy seeing two or more hot women get it on, but when I consider it, I get it. They don't want their woman with no one else but them.
But why settle for one scoop of your favorite ice cream when you can have two, or more...
As for lesbian content in this game, make it optional, problem solved.
If someone is so hard-core anti-lesbian they don't support the product because there's optional content, well, that's their choice, but what a poor unfortunate, miserable soul...
I'm 100% pro-lesbian content, as long as the women involved are hot...

And let me watch and eventually get involved...
By definition, being cucked or being a cuckold just means you got cheated on, irrespective of who did it or how, so a female can cuck a guy, hell a dog could if you have a story with that in it. The ability to knock up the female has nothing to do with it. Just to be clear.
Also, two scoops of ice cream is only better when you get to eat them both, not when they are in another room eating each other. The growing "anti-lesbian bias" that is happening in the hobby is in no small way in response to that. The big problem is not really that in 3-somes or group scenes *some* girls might touch each other a little, but that more-and-more it was full lesbians off doing their own thing and the Mc just catching glimpses or hearing about it and that still being treated as "lesbians are so hot!"
For a lot of people (when it comes to AVNs) any sex acts that don't involve the Mc are either useless or a even a turn off. Voyeurism is a fetish and being turned on by random lesbians having sex and watching it is a sub-set of that fetish and more people are realizing that assuming everyone enjoys that fetish and should just be okay with is not the way to go.
And one final thing I'll say about the "just make it optional," is that such an argument is flawed. Even having the option changes the characters involved in fundamental ways and depending on execution it can ruin a character to even have the option.
To illustrate this, let me give an example:
Say that the Mc walks into a room and there is a girl sitting with a dog next to her, and she says, "Hey would you like to see me and this dog fuck?" Even having that option has likely ruined that character, she just revealed that she wants to do that, it's a desire she has, and you just kept her from it and it begs the questions... is this something she has done before?
Maybe it happens a different way, like this:
The Mc walks into a room and sees a girl and a dog just being normal, she's watching TV and the dog is just sitting next to her, and the Mc thinks, "Do I want to tell her to fuck that dog?" (Even worse if he thinks, "It would be so hot to see her fuck that dog, I should make her do it.") Now, this way of implantation hasn't ruined the girl, but it has fundamentally put a barrier between me and the Mc, because that is a thought I wouldn't ever have. Now the Mc is someone who has been ruined and we are at a odds.
That is an extreme example to some, but there are some people who are just as grossed out by lesbian sex as they would be by bestiality. And you can apply this to many fetishes, scat, NTR, necrophilia, piss play, loli, shota, futa, gay... even the optional inclusion of some fetishes can deeply change the characters involved and most games don't implement optional fetishes in a way that avoids this.
The only way I've seen that can help avoid optional content deeply affecting the characters and story is having an option at the start of the story and if you say no to it then there would never be a single hint in game of even who it applied to, let alone signs that it happened behind the scenes. But even some games that go this route still have the signs of where it would have fit if you had said yes.