The simple facts here are:
1. Mozilla has appointed as a CEO someone who not only doesn't support same-sex marriage, but who has actively contributed to the campaign against it.
2. The CEO is the public face of Mozilla, and to some extent sets the agenda.
3. There are quite a number of developers and others in the community, some of whom work at Mozilla, who are either in or will consider a same-sex marriage at some point in the future.
4. These people are disappointed (reasonably so) that a person who is campaigning against their civil rights has been appointed to a relatively important and visible position.
5. These people, and other organisations and people within the community who support them, are making a clear public statement that this appointment is a problem for them.
That's the extent of it, and it's perfectly reasonable. I agree that there's no reason to hate or take this argument to extremes, but it's completely reasonable and fair to take a polite but firm public stance against this appointment.