Theoretically, yeah. There are tool using species right here on this planet that pass a lot of tests about self awareness, but we're an order of magnitude more intelligent than a bonobo (well, mostly) and shouldn't be banging them. I purposely used a species that has recreational sex as an example here.

If there were an intergalactic species that saw us as little more than self aware tool users, that would be up to their morals.

Either you're smart enough to understand the concept of consent or you aren't. Even if there is a sliding scale of intelligence of creatures (I have some issues with the concept of intelligence as a thing that can reasonably be objectively measured, but we can put that aside for now), whether you're intelligent enough to understand what you will be doing and what will be done to you and make an informed decision about whether you want to do it seems a pretty binary thing, and largely independent of your intelligence in other contexts (e.g. ability to use tools).

You could make the argument that alien sex might be complicated enough for whatever reason that that the aliens would argue that humans couldn't meaningfully understand everything that was being done to them and thus could not, by definition, give informed consent, but at least from my experience, humans seem pretty on board with things they don't fully understand. Whether it's ethical to fuck someone who has admitted that they don't know anything about what you're going to do to them, is aware they may be harmed by it, and wants to do it anyway is another question.

Besides, all we'd need is for one human to fuck one alien for science to know whether or not it was safe.

I don't think so... Intelligence isn't required for consent, it's informed willingness in the absence of coercion or manipulation. The other factor is the likelihood of harm - I think that covers the full ethics of what beings you can ethically fuck

There's a few ways to look at it

One of my parent's dogs once had sex with a handyman's foot to completion... By the time he realized what was happening it was over, and the dog wasn't very bothered. The guy was also very nice about it, but I bet he stopped wearing sandals to jobs

What if he was into it? It's not like the dog would care... He clearly knew what he was doing. If we're just horny dogs humping their feet, it's not like they're doing anything wrong by not kicking us off

It also reminds me of a YouTube skit where an 18 year old boy goes to an adoption agency to find a 16 year old girl to adopt, with clear implications. It's a dark joke, but I think it lines up with this nicely

Let's say there's an alien species far more advanced than us. We're too dumb to even understand the basics of their technology, so to them we're basically pets - they can keep us around, but we're at their mercy. Now, there's an issue - a human could believe, even falsely, that we have to have sex with them or there will be consequences. We could even believe it's expected of us - and that's not coercive

Now let's say that same species came to Earth, guaranteed the health and happiness if every human, and made tinder for humans who want to have sex with an alien. In that case, if you handle it correctly, you can minimize or eliminate coercion from the act

Maybe the aliens don't even have sex... Maybe they just think humans are cute, and they enjoy making us feel good the way we enjoy petting animals

I don't think intelligence plays into any of this - consent isn't about intelligence at all. It's about willingness in an uncorrupted state

Parrots don't have human intelligence tho.