they didn't say we should have sex with slave but those among the captives or slaves they wish to marry and they must marry them before having anything to do with them.like how prophet ibrahim marry hajarah.they dont have unlawful sexual act with slaves or the captive.they marry them if they wish and they convert to Islam when they see the beauty of islam.some christian mind are dirty and they view islam has a bad religion that does not have respect for a girl,woman,lady.instead of them to research and findout the truth,they will believe in heresay.

it will be better both of you call it quit and remain friend because her parent will not agree to it since she is their only daughter. in Islam,a Muslim guy can marry a christian girl is allowed in islam but a christian guy can not marry muslim girl.it is forbidden.if her parent are devoted muslims,they will never accept you.though is not going to be easy but you have to let go because if you force it,it will back fire even if you run away or impregnant her.her parent will not be happy with her and she will also lose them.i have see something similar to this before.