Marriage is an institution. Any institution/system can only be stable if its member follow certain common rules. Better if the rules are in harmony with the human nature.
Can an institution (say a school) function properly if each student arrives in school as he/she pleases? Won’t a game fall apart in allowing one player to hit freely while prohibiting the other, “below-the-belt” shots?
So, what if a Muslim marries a non-Muslim? Well, while a Muslim will have to comply with the Quranic law, his/her non-Muslim partner would be free to act according to his/her whims. Imagine a Muslim keeping his/her gaze low, avoiding liaisons and alcohol, while the non-Muslim partner returning in the night and declaring happily that he/she got drunk in the night club and kissed a hot guy/chic!
Hence this is the mercy of Allah that he put like-minded righteous people in each other’s company and prohibited them the company of non-believers. Imagine the anguish of a righteous person while his/her partner commits all kinds of sins and shamelessly calls himself/herself “free” or “open-minded”!
Isn’t it really heartwarming that your partner is completely yours?
"Tell the believing men that they should reduce/lower their gaze/vision and guard their private parts..."~~24:30(Quran)
"Tell the believing women that they should reduce/lower their gaze/vision and guard their private parts..."~~24:31 (Quran)
In another verse Allah says: And do not go anywhere near adultery, it is an outrage and an evil path~~17:32 (Quran)
How would you feel if your husband uploads the video of lovemaking on a porn site? Embarrassed?! Islam however, has made husband and wives garments for each other. That is, what’s between them, is concealed from the world. How beautiful!
They are clothing for you and you are clothing for them…”~~2:187(Quran)
Said this, existence of those who call themselves Muslims and yet commit illegal sexual intercourse cannot be denied. However the point is, Muslims have a moral code by mercy of Allah while non-Muslims do not have any such moral code. They are morally blind. A blind may hit the target, but that doesn’t mean he/she’s aware about what to aim at! Hence even the moral non-Muslims, are moral merely by convenience. That is they may relinquish their morals anytime, and start following their base desires; it depends on the call of their convenience. Non-Muslims do not have a moral compass. Morally, they are directionless.
A Muslim however is moral by choice-less-ness rather than convenience. He/she doesn’t has a choice but to be moral, because he she believes that eventually they will meet Allah and are answerable to Him. There’s no escape.
Every soul will taste death, then it is to Us that you will be returned~~ 29:57(Quran)
Quranic verses that prohibit the marriage:
Do not marry idolatresses unless they believe; a believing woman is better than an idolatress, even if you like her. Nor shall you give your daughters in marriage to idolatrous men, unless they believe. A believing man is better than an idolater, even if you like him.~~2:221(Quran)