I really do not understand what do u want 2 say. Do u imply that if we do so, then we r on our way 2 hell eternally even if we r in a true correct orthodox marraige ? Is there any evidence from the the Bible ?
No one is saying that anyone is on their way to hell here... judgment belongs to God not to us! What we're saying is that the act you've asked about is in fact a sin, and rather than leading you to intimacy with your spouce and with God as do marital relations, this act will lead you away from this intimacy and is actually a source of isolation, both from your spounce and from God. It is a selfish act rather than a giving one. No one is going futher to say that this seperation will be an eternal one leading to eternal condemnation in Hell, God is the judge of such things, not us, we all sin, and God knows how He deals with us according to His mercy. But it would certainly be safer not to wilfully commit such a sin, and besides, who cares about preoccupation with Hell? life is better in the love of God, and that's what we should persue, seeking His will out of love for Him, not fear of Hell.
As someone else said, all the unnatural acts that spill the seed so that lust can be satisfied without the marital act taking place go together: oral sex, anal sex, onan's method, masterbation, etc., none of these are the loving, giving, natural, sacramental, beautiful marital embrace that God has ordained to give grace to copules and to be a type of the love between Christ and His Church. Rather they are perversions of human sexuality.
That's not to see any of us are judging you if you do these things, they are difficult, addictive sins that are easy to fall into, especially in a society that trains us to think they're normal when they're not. It isn't the place of us sinners to judge you... However without judgment, and out of love I tell you that these things are not pleasing to God, and that you will be happier without them.
Lust consumes, it never satisfies. It is depraved, and just leads you wanting more and more, less and less satisfied. The satisfaction it promises is a lie it wispers before leaving us empty. It is completely different than the loving marital act (not that this too can't be done out of lust rather than as an expression of love if you're hearts desire is to take pleasure from your spouce rather than to love her).
From what I've read of these things from holy men who have experienced marriage: Basically these lustful things are immature, perverted expressions of human sexuality. In marriage, in the beginning if you need sexy clothes and that kind of thing, it's not wrong, but it's also immature (that is not a judgment or saying it's wrong, just that there's better to be had). Later marital sex should progress to be less and less about the physical and more and more about an expression of love... Not that you stop enjoying the physical, but rather that you discover more and greater depth of meaning in the spiritual and emotional connection... Although of course never should the physical aspect be despised or thought wrong, it is given to be enjoyed.
As for Biblical proofs, looks as people have mentioned about any instance of perverted sexual acts in the OT, or about what St. Paul says about lust and about sexual imorality in the NT... or about the types given of Christ and the Church.... there's lots to read there if you want to look, but I for one am not knowledgable enough to guide you through it, and really, if you want to you will read into it whatever you want to see, which is why you're better to subject yourself to the guidance of your father of confession rather than to make yourself your own father of confession by trusting in your own mind to interpret the Bible and discover what's right in a matter in which you have passions and desires.