Is so called "marital rape" consensual since they consented to marry?

A contract where one party permanently gives away all rights (love, honour and obey) seems deeply worrying, but OTOH, I don't think people should take serious vows so lightly. Maybe a middle ground would be that a spouse can refuse sex, but this is grounds for divorce and they have no claim for alimony because they broke their half of the agreement? Either that, or don't use the traditional vows.

What if someone says yes but feels pressured to?

What if someone only robs a bank because of peer pressure? Still guilty.

If two drunk students had sex, has a rape occurred?

It depends exactly what 'drunk' means. If someone is paralytic and can't actually articulate the word 'no' then its rape. But drunk sex is perfectly normal, and if you consent to getting drunk then you are responsible for your actions while drunk.