If you're a scrupulous individual, and the post makes it sound as if you are, if you're not SURE that you consented, it's unlikely that you did and a confessor would advise you to act accordingly (i.e. not refrain from Holy Communion, etc.). Generally speaking, unless you're very lax, if you "don't know" if you consented, then you haven't. If you consented with your will, then you'd pretty much know that you did, since the will doesn't operate without your knowledge.
Where it can get tricky is that you might exhibit some weakness in resisting the temptation, where your will isn't strongly rejecting it, or there's some degree of partial consent, perhaps an impure curiosity, perhaps some aesthetic but not quite so-called venereal delight, short of the full consent. But those situations would typically entail venial sin, not mortal.
IMO, unless you're extremely lax, you can't commit a mortal sin without knowing it and without being "sure" that you committed it.