Suppose what you saw was really extreme? Surely you knew there was a risk of that happening. You were willing to take the risk of seeing such a thing. So I’d say yes, it was a mortal sin, not because what you did see was genially sinful, but because you were willing to expose yourself to a strong temptation. If your motive was curiously about what acts different words referred to, and there was no need to know those things, why not maintain your innocence?
I don’t think you intended to commit mortal sin, rather, you omitted to think out your actions. IOW, malice on your part was not present, but the devil got you to take the bait.
So, don’t take Communion if you can’t get to Confession before tomorrow, but do make an Act of Contrition and decide what you’re going to do when this type of temptation comes on you again, and it will!
It’s best not to educate yourself on matters of this nature unless your state of life or profession requires it. ie, If you are counseling people whose problems concern these matters, your child talks about a sɛҳuąƖ matter that you don’t know what it is, the terminology appears in a legal case that involves you...
I’ll give an example where I had to learn the name of a certain impure act. I had to testify in family court concerning a child in my class who was being sɛҳuąƖly abused. Yes, I looked up on line for a dictionary definition of it, ie. just text, no pictures or depth necessary. I’d heard the term before, knew it was something perverted, and decided NOT to educate myself, but then I had to learn the technical word in order to help the child.