I think there is a degree of implied consent in a relationship. This does NOT mean that rape is any less bad in a relationship, or that either party loses the right to say no at any time. I do think that the bar for active consent can be lower though, so things like initiating when drunk, half-asleep, etc would be sketchy/illegal in some places with a stranger, but not really an issue in a relationship. Kinda analogous to groping your wife in the kitchen vs. groping a stranger. Both could be sexual assault in some contexts, but the former is not a problem in most contexts whereas the latter is also OK in some situations, but definitely more risky (both morally and legally). In an ideal world you'd discuss it beforehand but in reality, hardly anyone does that. You have to use your judgement a lot of the time to figure out where the line is, both in a relationship and with strangers. "Passed out" seems like an obvious line to most people (I'd put the line earlier than that, but later than "tipsy"), although people have sex with passed out people often enough that it does need to be pointed out.
I don't think it's fair to say that being drunk prevents consent in all situations, but the line gets murky enough that you should proceed with caution.