I still watch the show from time to time.
In terms of Ectofeature, there is reason to believe that such a relationship would be unethical. It's unfortunate that this is not clearly communicated to the audience, and in fact I would argue that there is a lack of care in developing the characters in general such that it probably did not at all occur to the writers this would be the case, as they did make a joke about BJC and Rajeev potentially being on a date in "Three's a Crowd" (I'll get to why that's bad in a moment).
First, Spencer and Billy are distant cousins. I don't have encyclopedic knowledge of the show and its official materials, and even if I did, records of these things are sparse, so I can't back this one up. It's important to note though because many fans are aware of this as truth, and thus view the ship as incest.
The second reason, one I can present argument for, is Billy is an adult and Spencer is a child. It's in BJC's trivia section that his official Spanish bio says he died at 19. The link does not work, however, and thus I can only assume the bio only exists as a screenshot on someone's social media account, or some such.
Not all is lost though, simply look to the episode "Ponzi's Got Talent", in which BJC is shown being a child in the same time-frame as Principal Ponzi. Even if we were to assume BJC was a few years younger than Ponzi in that scene, Ponzi himself is significantly over the age of 18 in the present, thus the same would be true of BJC. (This is why his age is listed as 19-27. We have reason to believe he died at 19 and has been a ghost for years. The 27 is perhaps too specific as there's not anything to concretely put him as being dead for 8 years, but I've yet to think of a better edit to replace that with, so there it remains.)
Spencer (and Rajeev, to connect back to an earlier point) are in high school. They are generally understood to be underclassmen, but even in the upper ranges of high school ages, it'd be dubious for someone of Ponzi's or BJC's age to pursue them.
