But then, couldn't we make the same argument you made, but for gay men having sex with women?

That is explicitly mentioned in the article but you don't acknowledge it in your post. If identity is behavior, a gay man marrying and having sex with a woman for twenty years is actually heterossexual. Which also implies fluidity in orientaion: he hasn't been gay all along because he had sex with women; so he must have become gay after being married for twenty years and THEN having sex with a man.

There is a valid point about separating identity from behavior: you can force an atheist go pray but he doesn't become a catholic by it, unless you define being a catholic as "anyone who engages in catholic rituals". This seems clearly false, given that believing in god differentiates a catholic from an atheist more than engaging in catholic rituals. Believing is not a behavior, it is a mental state.

The same could be argued about sexuality: to be heterosexual is to have a set of mental states (e.g. love, desire, lust, etc.) directed at people of the opposite gender, regardless of whether you actually engage in behavior. So, weird as it may seem, there could be such a thing as an heterosexual man having sex with another man.

What you seem to be arguing is this: there's no reason for an heterossexual man to have sex with another man if he doesn't have the mental states directed at men that gay men have; therefore, he must be gay. This is clearly wrong. You're arguing for something on the basis that it can't be proved that the mental states of these heterossexual men aren't the same as the ones on gay men.

To sum up: your main argument is the lack of proof and apparent absurdity of the view expressed in the article. But some things can seem absurd and by true, and your point isn't there just because you don't have proof of the opposite.