Can you argue the same thing about e.g. Mirror's Edge? Is it and it's fans inherently sexist against men because there's no male option?
Players actually avoiding a game for offering female characters as an option would certainly be sexist of them.
That sounds like a real grasp at straws there. Is it that common to say e.g. "I'll never play Mass Effect because there's the
option of playing FemShep". Not to my knowledge, they just played MaleShep and ignored the other choice.
Mostly, the single-gender games don't offer you a "choice" at all - there is only one main character and the entire story is built around that character. Its majority rule in those situations - if
51% of your potential audience respond better to making the character male, then you make the character male. Adding options to a game that isn't built to support options has more costs than just throwing in a new model or skin, you have to create whole new dialogue and situations to support it. Female Kratos for example, you'd need to do more than just make a new skin, you need to create a whole load of extra dialogue to support the option. Plus, any time there are a lot of animations / movements for the main character, you'd need to recreate and test all of those separately for a new model.
"Team" games and multiplayer games are a lot different to games where there's only one character and it's built around a single-player narrative. Those multiplayer and team-focused games make up a lot of the games in the 50% of games that offer choice of protagonist / gender.