Is playing game with sex same as commiting adultery, there was a game I used to play which had anal sex and I used to play it, so am I accountable for that sin even though I am sure I will try my best not to do that in real life, and another question is anak sex kufr?
Yes. It is adultery. But adultery has different levels. Just like there are major and minor sins.
Abu Huraira reported Allah's Messenger (ﷺ) as saying. The adultery of the eye is the lustful look and the adultery of the ears is listening to voluptuous (song or talk) and the adultery of the tongue is licentious speech and the adultery of the hand is the lustful grip (embrace) and the adultery of the feet is to walk (to the place) where he intends to commit adultery and the heart yearns and desires which he may or may not put into effect.
Sahih Muslim 2658
Here is another evidence. Clear command ordering muslims to not look at forbidden things.
“Tell the believing men to lower their gaze (from looking at forbidden things).” (24:30)
Answering your last part of the question: This is not from the actions of Kufr. Major sins do not take anyone outside the fold of Islam unless he/she commits shirk.