It's fairly unlikely to be accurate but not just for the reason that you've suggested. If you think of sexuality as a spectrum then there's likely to be a number of people who identify as heterosexual who are aroused to greater or lesser extents by pornography that is stereotypically gay - that doesn't make it a good test as to whether someone is homosexual or not.
Then you've got the issue that not everyone shares the same sexual preferences, some people like this physical characteristic while others like that physical characteristic, some prefer X personality trait in their partners, some prefer Y personality trait etc etc. It's not that there won't often be some overlap in what arouses people but as a test for "proving" someone is homosexual it's flawed.
I'm not sure what PPG is in this context but I'm far from convinced in the general principle as per the above. It's a big assumption to believe that all people respond to the same stimuli, particularly in something as convoluted as sexual preferences.
Fortunately (or unfortunately depending on your point of view) we've signed up to conventions on human rights and the right to liberty and security, freedom from discrimination and private and family life. If we're going to claim any moral authority in the world then we've got to be prepared to back it up. What if you had to hide your sexual orientation - how would you feel about that?
So even that system which manages to be humiliating and demeaning is subject to abuse.
The problem is that as much as 5 to 7 % of the population is thought to be gay.
Can we really take in that much of Africa? Why should the few that have been able to pay huamn traffickers be allowd in? there are many many safe countries on the way to the uk but they do not seem to be running there? It's safe enough in india and very easy to slip into india from Africa without anyone knowing or caring. Do they go there for safety? No. Because they are economic migrants who are lucky enough to have a way in.