Either one of those (or any reason really) is not a good excuse for making a statement like that in any form. Once the news got out that Assassin’s Creed Unity Co-Op will be a boy’s only club (in terms of the characters) the media got a hold of it and the storm began. Ubisoft has responded that they are not being sexist, but since they expect everyone to play Arno (the main protagonist) there was no reason to make any female avatars for the game.
If you believe Amancio it would have been twice the work to add that “feature” into the game one good quote from him is:
“It's double the animations, double the voices, all that stuff, double the visual assets”. He follows this one up with: “the only logical action we had was to cut the female avatar”
This is kind of silly when you consider that Ubisoft was pushing the fact that the game world is more open. Large and a bigger experience than any of the others before it. I would think that there might have been room to drop a lady or two into the game without too much trouble. In conversation with a couple of developers that I know they seemed to feel the same way. Although, they did say that it is roughly double the work, the actual amount of work was not all that significant and certainly not enough to require them to be cut out.
Personally I do not think that Ubisoft is at all sexist, I think the people in charge of production and development are just a tad foolish. To think that this move would fly for a game of this size and visibility shows a lack of understanding of their audience and the market as a whole. I do not think it will impact sales in a major way, but it could be the start of a decline in the game’s popularity (certainly with female gamers). When you screw the pooch this bad it does not go unnoticed, or uncommented on.