It would have to be something you did with full consent of the will, i.e. deliberately and freely. Compulsions and habits can mitigate guilt. If you looked at it a second time, giving yourself over to the act after sufficient reflection and saying to yourself I know this is a sin, I know this is grave matter, but I don't really care, I choose to do it anyway, then it would probably be a mortal sin.
If it was a compulsive, impulsive act in the moment, then it wouldn't be.
Mortal sin requires, besides grave matter, full consent of the will. Acting in the passion of the moment, or without sufficient reflection, impairs such consent.
Also being tempted is not a sin. Deliberately, knowingly, and with sufficient reflection putting yourself in the occasion of temptation intending to sin would be mortal sin, but if temptation is accidental and not intended, it is not a sin.
Subsequent glances can be a stumbling block and hence problematic, but if they come out compulsion or passion or addiction, it is likely not a mortal sin.
- For example, suppose you saw it once, were captivated by it and became briefly obsessed in the moment with studying it because it was arousing. That would not be a mortal sin.
- But if you saw it, went away, and a few hours later, after cooling off, thought about it and made a decision to find it again and inflame your passions, that could well be a mortal sin.