drunk sex has been taught as consensual, however as we have progressed so has our understanding of consent.
What (technically) makes their 'first night' rape can be explained by the following:
Ripley cannot remember the night at all, proving that she was drunk to the point where she could not have consented willingly (even if the "initiated it"). If she was lightly drunk/tipsy and thus could remember, it would be more debatable since that could have meant that she was more able to consciously consent.
So you can drink and have consensual sex, but if that person is so drunk that they throw up or black out, that is a sign that they are not at all conscious enough to consent to anything and thus any sexual activity with them would be non-consensual and thus rape.
Also, we can tell clearly based on her behavior after said "one night stand" that she would not have wanted to do that if she were sober. One might argue "but she fell in love with him so it couldn't have been" but at the point of time that they had sex and for a decent period of time thereafter, she clearly wanted nothing to do with the duke nor have any romantic/sexual relationship with him.
He loving him, whether she expressed that or not, doesn't clear him from that rape. To use real world examples: let's take a in-love married couple. If either spouse were to have sex with the other when they expressly said they didn't want to (or were similar to Ripley and unable to consent), that would be considered rape

Not only did he r@pe her but he does this shit and y’all justifying it saying its karma for not wanting to marry him