**a situation where you personally are very likely to fall and commit a mortal sin **
I don’t understand this. If I personally make the decision that I am not going to have sex with my girlfriend, how is any situation “very likely” for me to then have sex with my girlfriend, and do it with full consent?
Because despite what your previous decisions and commitments are, when the temptation arises, it is easy to set aside your previously made commitments and instead give in to the temptation and sin.
I have personally made the decision that I am not going to have sex with my fiance. That doesn’t change the fact that certain situations can tempt me to give up on that resolve. The same goes for you and every single other person in existence. Now, I can’t say exactly when and in what situations you would be putting yourself in a near occasion of sin, only you can know that through honest reflection, but to knowingly put yourself in an occasion of sin without a good reason is itself sinful because, as I said, you are being reckless with your soul. Sleeping in the same bed as someone you are romantically attracted to is in the vast majority of cases, a near occasion of sin for people.
Another factor to consider is scandal… (leading other people to sin by example). If people know that you sleep in the same bed they are going to assume that you are sleeping together, and this can lead to them becoming more accepting of the idea of sex before marriage and could lead them to eventually give in and do the same. (This is especially true of Catholics and Christians… if even the
Christians/Catholics are doing it, then it
must be ok nowadays, right?)
Even if you tell them you are not sleeping together (and they actually believe you about that)
andyou personally are able to sleep in the same bed as your girlfriend without
either of you putting yourselves in a near occasion of sin, the vast majority of people cannot. Which means your example could lead other people to try the same thing, ending in them failing and sleeping together. And once you sleep together once it is harder to avoid it in the future.
Now, both of these, near occasion of sin, and causing scandal, are not things that must always and everywhere be avoided, if one has a serious enough reason then one can perform actions which will put them into temptation and/or lead others to sin, but I really don’t see how showing your girlfriend affection in this way is so necessary that it makes it ok.
In summary, even if this situation is not a strong temptation for either you or your girlfriend (which is unlikely) you are most likely helping weaken other people’s resolve from waiting until marriage which is wrong becuase it is uncharitable to others.