I think you nailed it although maybe it was because the devs are men? They are thinking, how can we men not risk being taken to court every time we have sex. Especially if we are or soon to be crypto millionaires fearing every drunk hookup will result in extortion.
Or they aren't psychologists, or smart enough to hire them, or even just, you know women who have a clue. The examples in app kind of speak volumes to hire little thought went into it. Seriously someone is going to consent to photos and video but no explicit language? Ummmmkay. Okay it was just an example but why are those things even on there?
To me the time for this is after... If you just had sex and you can't even get someone to indicate it was consensual you clearly have a problem. Obviously that has to be done in conditions that clearly show it was without duress. No gun been pointed or a big pile of cash nearby. I'm not sure that can really be certainly decided but it raises the bar.
In many of the sexual harassment cases that have come to light there seems to be reports like "well I apologized afterwards and she accepted". Seems like memories may be a little fuzzy here, I reckon what guys think of acceptance may be rather loosely defined. Having AC post coitus declaration might help things a little. Indeed having people make somewhat binding declarations might help even though as I indicated it's really hard to be sure something was above board even if the party signs something. How many witnesses get badgered into signing a completely false concession? Then how bad does that look and might end them up wrongly in jail.
Maybe if you want real evidence then the whole hookup is recorded but neither party can access that without both parties consent. If it goes to court that will be contingent on releasing the recording. That's what the pre coitus agreement should be about. So it becomes would you agree to hookup with a blockchain as our witness?
Guy could refuse to release his key and take the fifth but how bad will that look? Or it could be a two of three key and the court or some other party has that third key. That would solve the problem of one party no longer being able to consent - maybe because he/she is dead. That requires more trust... You could make it require a court order witnessed on the blockchain for the third party key to be released. So a lot of hurdles but possible.
I do think the blockchain has uses to record legally binding contracts between parties along with A/V records to indicate parties understood the terms and it at least had the notion of not being coerced to the limits that is possible. No violence, no altered states of mind that kind of thing.
Now there is a place for blockchain to help out with sexual relations... Thats for sex workers dealing with payment and reputation issues. And there are solutions to that being built already (see intimate.io and Rendevu). But these are much more business focused and not casual hookup.