Think about it in these terms. If a relationship equals instant consent, can she use your money or bank account without clearing it with you first?
After all, if consent is assumed then this wouldn't be problematic.
Think about the spirit of that scenario and apply it to something even more sacred: someone else's body.
A relationship is an indication that someone loves you and finds value in you as a mate. It does not mandate the dissolution of personal autonomy or choice. It certainly does not give someone free reign over someone else's body. We have to learn to not take people for granted.
Keep in mind if this has been established then it's completely fine. I personally never had this issue and had dudes ask me in the past if it was okay. I actually like being woken up for sex b/c my sleep is actually better after an orgasm strangely enough. I'm also weirdly most horny in the early morning. So getting fukked awake can be so satisfying, especially if it's a lazy weekend and you can doze back off...
But all of this was communicated (and laughed about when our tired asses would fall asleep on each other at the end of a rough day trying to fukk sometimes

.)
Funny story time, me and one of my exes once did a favor for some of our married friends. We watched their 1 year old and their 7 year old twins.
At the end of the day, we were so exhausted but still trying to get it in. Snores in between moans..."a-are you sleep after you...?!"

Tangent, but always communicate first is the moral of this long ass post.