Afraid of marriage out of fear of committing sin

It is emphatically forbidden to have intercourse with one's wife in her back passage, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever has intercourse with a menstruating woman or with a woman in her back passage, or goes to a fortuneteller and believes what he says, has disbelieved in that which was revealed to Muhammad.” Narrated by al-Tirmidhi, 135; Ibn Maajah, 639; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

And he said: “Cursed is the one who has intercourse with a woman in her back passage.” Narrated by Ahmad, 9731; Abu Dawood, 2162; classed as hasan by al-Albaani in Saheeh Abi Dawood. See also question no. 1103. 

With regard to your avoiding marriage because of these fears, this is dealing with one mistake by making another. Ignore this waswaas (whisper from the Shaytaan) and keep busy with things that will benefit you in both spiritual and worldly ways. Ask Allaah to purify your heart and protect your chastity; hasten to get married for that is the best remedy for you. Beware of the wrath and anger of Allaah and of His painful punishment, and do not be one of those who are heedless. 

Remember Allaah a great deal and do lots of acts of worship, lower your gaze and avoid looking at haraam things, and avoid committing sinful actions, so that these evil devilish thoughts will leave you. 

With regard to coming to your wife from behind but over her clothes, you should avoid doing that. Do not be like a shepherd who grazes his flocks around a sanctuary and soon transgresses its boundary, as the Prophet (peace and blessings of Allaah be upon him) said. (Narrated by al-Bukhaari, 25; Muslim, 1599). 

Ibn Rajab (may Allaah have mercy on him) said: 

Allaah has guarded these haraam things and forbidden His slaves to approach them, and he calls them His limits. Allaah says (interpretation of the meaning): 

“These are the limits (set) by Allaah, so approach them not. Thus does Allaah make clear His Ayaat (proofs, evidences, lessons, signs, revelations, verses, laws, legal and illegal things, Allaah’s set limits, orders, etc.) to mankind that they may become Al-Muttaqoon (the pious)”

[al-Baqarah 2:187]

Thus it is clear that Allaah has defined for them what is permissible for them and what is forbidden to them, so that they will not approach what is forbidden and they will not go beyond what is permitted. Hence Allaah says in another verse (interpretation of the meaning): 

“These are the limits ordained by Allaah, so do not transgress them. And whoever transgresses the limits ordained by Allaah, then such are the Zaalimoon (wrongdoers)”

[al-Baqarah 2:229]

The one who grazes around a sanctuary and close to it will soon enter it. The same applies to one who goes beyond what is permitted and falls into doubtful areas. He approaches something haraam with the intention of merely coming close, but soon he will commit forbidden actions blatantly. This indicates that we must keep away from haraam things, and erect barriers between us and them. 

Jaami’ al-‘Uloom wa’l-Hukam, 1/208 

May Allaah protect us and you from all evils, and make us and you content with that which He has permitted. May He give us enough of what is permissible that we have no need of that which He has forbidden. 

And Allaah knows best.