All sins are common to man , if you think about it every man fap 
The thing is people would marry young age and have sex but we went towards hunting money and carieer so we delay mariage exchanging it for money and fornication. Women would be ready to marry when they reached thier first period . God did not create man to get degrees and finish school but to have dominion over earth and animals , Biblical mariage was planned and husband would take wife into his room added for that exact purpose in his house .
Releasing seed is not sin in itself but more likelly watching porn and lust thougs . Your body will automatically release itself when you sleep if you do not have sex for about 2-3 weeks .
Also the sin of Onan was that he did not fullfill his obligations , if brother's wife had no child and his brother died , he was supposed to impregnate her and take her as wife so her child would inherit all land because only son could inherit father's goods it could not be daughter (untill one Guy asked for permission for his daughter to inherit long after ).
So Onan was not punished with death by God for spilling seed on ground but for taking Wife with all these goods . He was greedy and wanted to keep land for himself .
Back to topic .
If something glorify God then it is not sin . If it does not glorify God it is probably sin.
Fapping is sin . But it is as bad as hating somebody or even cursing .You just feel like it's worst sin than saying a lie or something like that because when you release yourself you lose dopamine .
For whosoever shall keep the whole law, and yet offend in one point, 
he is guilty of all.
Telling a lie is as bad as murder in God's eyes .
We sin multiple times a day that's why we need to be saved .
Get wife problem solved 

If not try to not watch porn and don't leave hands on your pants because you can be touching yourself not even realising it untill it's too late and you are in "lust mode " .
Same goes for girls they often touch thier breasts while watching tv or something .