In Berserk manga, the very first scene is a sex scene involving Guts and one of the Apostles. Although that scene was what got me into reading the manga, now I'm not sure I find much sense to it. From what I now, Guts hates the Apostles. If he knew that woman was an Apostle, why would he have sex with her? Why not just kill her before she turned to her "Apostle form"?
Having a fight with a powerful Apostle is risky, exhausting and difficult. Putting his Dragonslayer and knives aside he gave her false sense of security. While she felt confident and let her guard down he fired a cannon into her mouth.
He knew she was an Apostle, his brand responds to demonkind. There was no other choice to make her vulnerable than to have sex with her.
Bottom line: he was aware and he did it to trick her and kill her the easiest way.