In case you are asking about the original post and not the follow up joke, its a reference to western gay culture, which may be a bit difficult to explain, as there's a few layers to it.

• there's a lot of code switching (essentially changing outward personality) between your friends, close friends, gay friends, family, etc, depending on what they are comfortable with/how uncomfortable you want to make them. Having multiple phone numbers would be a way of separating the way of speaking based on what number you are texting from
• multiple phone numbers is also a good way of separating you personal/work/sexual contacts
• there is also a culture of both blocking your flings on all social media/texting/etc, but then making alt accounts to stalk them/check up on how their lives are going. Having multiple phone numbers could help with authenticating those alt accounts, and also gives you a way of bypassing their blocks on you for either harassment or apologies, or even just to ask for a night out with them (even if its specifically to spite a 3rd party, and not because you wish to make up with the person you blocked)
• 'X thing is homophobic because it inconveniences my gay ass in this moment' is a popular meme (as pointed out by my spouse)
• probably another layer I am too autistic/not enough in the community to get
• honestly, my first few points are probably very much over thinking it