I don't understand his dialog the way that he wants Tav to have other lovers and more like he isn't opposed if Tav has other lovers beside him. Because he himself might take another lover while he is in a relationship with Tav too. Right now, all he wants is Tav, but this might change. And if Tav doesn't like it I guess this means break up because he has to follow his "nature".

If you select the option to reject him after you admitted that you feel the same but your nature is different to his nature (meaning you want monogamous relationship and not what he wants) there is a bug - he will first react to the rejection (saying he had to try at least) but the next day he will great you with the normal romance dialog, calling you lover etc.