I think that really depends on your own risk preference. I personally would not want to have unprotected sex with someone who as a default has sex without barriers and I might need more precise communication how we would practice safer sex. If its relevant think I would also want to talk about contraception and how they would deal with (unwanted) pregnancies in their relationships.

But I dont see anything unethical about this assuming this happens in the context of informed consent and without weird power inbalances.

Is it sustainable to only have one?

I think that really depends on you and how you want to go forward. I can see how this might become an issue in situations where having barrierless sex is important in more than one of your relationships.

(I just realised I'm responding to a post that's a month old but I already wrote this so here you go)

I think it's about what you want and find comfortable for yourself. For me, I've been poly for like 12 years and it's wild how the community has changed. In 2011 people thought the idea of coming out to your family or work colleagues was absolutely ludicrous (someone on the polyamory subreddit c. 2012 told me it was like telling my colleagues I like anal sex, fun times). Now it's pretty well accepted as something you should expect to do.

So too have the feelings about barrier free sex. In 2011 it was a given that you'd only have one barrier-free partner and that you had an implied right to know their level of barrier use with their other partners. Things have relaxed a lot, and rightly so because there's a lot of stigma around STI that is completely unearned as they are all highly treatable if you have the financial means and caught early (yes, even HIV).

So now, I have barrier sex by default with new partners and the conversation about changing that happens organically. They know what my risk tolerence level is, and I know theirs, and we mutually can make a decision about whether our risk profiles are compatible.

I use a condom with everyone and I think it's insane people don't. I don't trust everyone in the world. I somewhat trust my partners but I certainly don't trust their partners, or their partners' partners, or their partners' partners' partners'.

But I can't control them or their decisions. Only my own. So I use a condom. Simple. Effective.

To me it all depends on the shape my relationships take, there isn't a one size fits all answer.

As it stands right now, I've have one serious partner. However, my partner has a total of 3 serious relationships. The way things are currently setup is that there is barrier free sex within committed, serious relationships (or that will be on the table at some point), but for newer relationships and/or flings: the number of condoms used needs to be equal to or greater than then number of penises involved.

While testing is important, I don't have too much to say on this. It comes down to communication and openness. I mean, while they're best avoided, often the stigma around the STI is worse than the STI itself (not universally true, I know).

As for the ethics of it: I don't understand why it would be unethical, as long as everyone is communicating, truthful, and on board with things.

Communication and transparency. Everyone should know so that everyone can make decisions based on their own risk tolerance.

Personally, in an open group, barriered sex is the default except for two primaries. In a closed group, I think it just depends on everyone risk tolerance and trust.

Personally I’d be fine going barrier free if I was in a closed group I trusted. Like all things, the bigger the group, the more likely somone isn’t playing by the rules.

So I think it depends a lot on the circumstances.

I'm navigating this with my partners right now.

I've decided to go barrier-free with my two partners, and use condoms with everyone else.

We are a pretty incestuous polycule and mostly use barriers with those outside the friend group, and we're all tested frequently. ~3 months.

Me and one of my partners also take daily doses of PrEP, which nearly eliminates the risk of contracting hiv. I'm also fully vaccinated, so the only thing I could potentially get that can't be eliminated with drugs is herpes, but that is very treatable.