You can avoid relationships and still have a harem. With Ziv just don't go to the pond on the Amazon warrior women planet after she puts the white water flower on your bed. Sex with Renae can be avoided through dialog as well. Same with Thyia. In fact you have to be extra pushy with her to have sex with her after the quicky when you first meet her.
I believe Celine will come around if you're truthful with her and propose a harem. She'll need to think on it. She won't be happy at first, but she'll come around.
And while I agree that Rahia and the muscular warrior gals are VERY appealing, they have their own planet, their own culture and see you largely as a warm, fleshy sex toy. You cannot recruit them onto your crew.
Plus, according to Perv, we won't be seeing Rahia again until Episode 3. The blonde gal who leads the priest class (I forget her name) makes appearances in Episode 2 but you cannot interact with her. I didn't want to. Her daughter, otoh....but sadly she's not in the picture in this episode.
I'm assuming of course that you have Episode One because you referenced the hot warrior gals.