It's not that one thing or another makes it okay. There are more neutral ways to phrase it that don't needlessly exclude vast potential audiences.

> Instead of scratching only your own itches, why not scratch your partner’s itch?

> Instead of scratching only your own itches, why not scratch your lover’s itch?

> Instead of scratching only your own itches, why not scratch your sweetheart’s itch?

> Instead of scratching only your own itches, why not scratch your mate’s itch?

Depending on who you ask, at least a plurality of men are at least a little bi, but a double whammy of homophobia and biphobia keeps them closeted. Why make them twitch a little when a small change in wording solves the problem? Being a good for-public writer is all about maximizing your potential audience without diluting the message. "Girlfriend" rather than any of a slew of more neutral terms needlessly dilutes the message.

Personally, I would cut the question entirely since restricting the message to partnered people doesn't serve any purpose.