mrsparker, for what it's worth, and whatever people think of your response to this situation, i think you have been quite brave to be so honest about the way you feel about this situation. in addition, i think it is vital that you are honest - with yourself and your dp - while you make a decision.
you are quite right, while your first responsibility will always be to your children, dp's will always be to his, so if you can't face the propsed future you may have to make some very difficult decisions.
also, your dss is only 6 now, but will grow up more quickly than you can imagine ( for all us parents). if he has issues with his sexuality and taking advantage of other children, this needs to be addressed by a professional. an enormous amount of damaging sexual abuse is carried out by older children - not just dirty old men. alternatively, as other posters have suggested, this might just be normal 6yo behaviour.
he is just a sad child who needs love and care and maybe help. even if he doesn't get that from you, could you ensure that dp seeks advice, maybe initially from your gp? i am sure you don't want to just wash your hands of him, and (again) as has already been pointed out, your youngest child will always be his brother so this is in your interest really.
i have to agree that it seems a bit rich that dp is prepared to live with your child full time, but you are not prepared to live with his. it must be v hard for step parents to see their dp's dcs over breakfast every morning and not see their own . he has surely made big emotional sacrifices?
but that in itself is not particulalrly helpful, because what you are saying, i think, is that you just don't feel you can do it? and if you really are not able, then maybe you have already made a decision? if that is the case, there is no point in us making you feel bad about it - presumably you already feel a mixture of awful and despairing and torn...
the only really constructive things i can think of are that ime truly loving dp tends to lead to loving dscs - because they are a part of each other iyswim? and that you should always always always be truthful and honest with each other - you need to both go into this with your eyes wide open, and keep the communication up throughout.
i hope that helps? i hope you don't feel judged. and i hope you are able to access the rl support you all need to make this work.
(had to go and do something part way through typing so if this has all been superceded by the time post appears, i apologise!!)