Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear questioner, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work sincere for His Sake. We, really, admire the honest approach you are taking.
We understand the frustration you feel at your partner’s sexual relationship with you as he knows this is against the teachings of his religion, and yet it is forbidden to marry someone who is not from the People of the Book. So, such a marriage cannot rectify the wrong he has committed.
As for what you said about people whom you see feigning belief but only following rules that serve their interests and your opinion that religion creates separation and discord among people, we would like you to analyze the whole situation with the same candor you display here.
As it appears in your words, you know that there is a Supreme Being who administers all the affairs of humanity. This Being is what all the adherents of heavenly revealed religions call “Allah”, or “God”, as you know it.
Moreover, there are virtuous men whose trustworthiness, chastity and righteousness are unquestionable. These men whom we know as Prophets and Messengers came throughout history with intermittent messages indicating that they were sent by a Supreme Being known as Allah, in Whose Name they preached to people. They called humankind to monotheism and confronted polytheism, atheism, and other forms of worship that go against the teachings of monotheism.
In responding to this divine message, people are divided into two categories. The first one includes those who wholeheartedly embraced this call and sacrificed everything in supporting the Messengers of Allah. The second category, unfortunately, constitutes the majority who refused the call of monotheism. They refused to grasp the light of faith and instead preferred to sink into darkness. Thus, they incurred Allah’s punishment. Their traces are still noticeable everywhere for the present and the coming generations to learn lesson from. Allah Almighty says, “Then see the nature of the consequence of their plotting. We destroyed them and their people, every one. See, yonder are their dwellings empty and in ruins because they did wrong. Lo! herein is indeed a portent for a people who have knowledge. And we saved those who believed and used to ward off (evil).” (An-Naml 27 50-52)
Given that you are a person of insight, do you think that Allah created humanity to wander on earth without any purpose or guidance?
Allah has great power to administer people’s affairs. His Might is not confined to creating them.
Allah instills in the hearts of all animals, whatever their species may be, maternal instincts towards their offspring. He created the whole universe in the most beautiful form, making the earth suitable for humankind to live in comfort. He also arranged everything in due proportion so as not to cause harm to humankind. We can, therefore, understand that this Graceful and Merciful Lord will never leave humanity, His creation, without guidance.
Besides being the Most Gracious Lord, He is the Supreme Just, in the sense that as He rewards for good deeds, He justly brings wrongdoers to account. His scale of justice is always in perfect balance. After sending His Messengers, Noah, Abraham, Moses, Jesus and Muhammad (peace and blessings be upon them all) to humankind to guide them to the Right Path, the normal path to follow is to assess their response to the Messengers sent to them.
Now, most importantly, try to imagine how the situation will be on the Day of Judgment when people are brought to stand before their Lord. Actually, it will be a horrible scene seeing people being driven upon their faces to Hell, while it will be pleasant to see people in very beautiful garments being led to Paradise. To which group you would like to belong?
All these points and remarks are just to induce a sense of reason and to call you to the Right Path. May Allah guide all of us!
Regarding your question whether a Muslim is allowed to marry a Deist, the answer is that a Muslim is not allowed to marry except either a Muslim female or Christian or Jewish one.
We hope that you adopt the path of Islam. Please keep in contact, and may Allah be with you.
Allah Almighty knows best.