+add as contact
I've never been in a mixed sauna where sex was allowed, only gay ones.
If someone touches you and you are interested, just touch them back, things will move on quickly from there. If you are not interested, ignore it or say "no thanks". If you are interested gently touch someone to show it and be prepared to take no for an answer.
If someone has their cock out and is stroking it at you, then you are in!
Be prepared to say no if you don't like something and take no for answer from the other party.
All rather straight forward.