Huh, I'll say, that's a little strange. I haven't seen many bad reactions to MareXMare shippings in this fandom. It's what most of the community seems to ship, and Fimfiction and Deviantart and wherever else is filled with it. Sure, you get one or two complaints, but I've only seen a couple.

 Usually, I only see disgust thrown at StallionXStallion, which is dumb because it's no different than two mares.

Anyway, those people are being ridiculous and you should ignore them, there's nothing wrong with being gay, and there's nothing wrong with just shipping two characters of the same gender either. Ignore them and keep shipping who you want.