Unless you and your partner have a discussion and an understanding that the relationship is open for both of you to have sex with other people, then of course sex or sexual behavior with someone of the same gender is cheating.
When people say "Oh, if it's with an other girl it isn't cheating." that thinking, in essence says "women don't count.
" If one has sexual behavior with same sex partners, they can form emotional attachments to them. It's easy to fall in love and even if you don't, you're sharing something with an other person that may interfere with and hurt your partner.
Without a Poly understanding between you and your partner any sexual behavior is cheating.