Expanding on my headcanon for Belgium's dynamic with Porteng (and a bit of hc about Porteng in general).
England & Portugal have always been a thing in many nations' minds (probably got together around the late 14th-early 15th century, so a bit before either really became big powers). Some of them never knew the two when they weren't together, even if the two have had some periods where the relationship was colder (during the Iberian Union and perhaps during the very late 19th).
Now, I don't think many of the nations particularly care that it is a homosexual relationship (I mean, look at most of them...). The issue would come more from the public. I would see England as someone who cares a lot about public perception of him (Portugal would be a bit more carefree). Now up to the 19th century, this relationship wasn't as much of a problem to him in regards to the public. But when news media exploded throughout the 19th century, I think he realised he had to become more careful.
They'd probably be considered celebrities (perhaps to the level of the Royals), so you can if anything happened with him, it could be reported in those media, including relationships (and even MORE since it's a gay one, they could even get in trouble). So by that point, England insisted on being discreet, while I'd think Portugal would be a bit more carefree (it could still be passed off as the media overreacting, or they could just claim they've grown up in another time and and are "shocked" that their perception of friendship is taken as something else..). But still, it is a problem that bugs England.
Comes in Belgium. She and England would have struck up quite a strong friendship after the Treaty of London. When we take into consideration that England at the time was in the Splendid Isolation era, and therefore there was distance between England and Europe, Belgium must have been (with Portugal) one the only friends he essentially had from the continent. The two have a strictly platonic relationship (England is into men in my hc).
Belgium would be very much aware of his worries with public perception, and decided to help him by teasing a relationship between the two of them as well. Now, while that doesn't remove the issue of public perception entirely, it would blur who would be England's hypothetical lover. So she'd become extra touchy, perhaps flirtatious with him in public (Portugal is aware of what the intent is, I haven't decided if it would be him figuring it out on his own, being told from the beginning or later on). You can see it as an attempt to cancel both possibilities out to the media, and therefore to the general public.
Now, none of the other nations is fooled, but it's not a strategy made to fool them, it's meant to fool the public and the media. As for the latter, if some still want to insist that Porteng is a thing based on their long connexion, others will refute it, and they'll have more concrete evidence (as Belgium is doing it on purpose to be seen). Belgium and England do keep it subtle enough, though, so it can be passed off as mere gossip.
Over time, there is less need to counter those rumours. But she still teases him from time to time, and the question of these two's relationship still lingers on. And you can bet with the rise of internet that there must be debates online from time to time when some tabloids decide to act up. Perhaps ship wars even ???
