Unmarried couple stay in 1 room?

I'm an American citizen and Muslim, local Islamic law applies to me. It's not if you're local or not.

I don't know if this will help because I am American and married a Chinese Malaysian and neither of us are Muslim.  The laws pertaining to Muslims are not applicable here regarding religious convictions and marriage.  However, we had made a trip to Indonesia for a couple of days before we were married and we were not allowed to stay in the same room, although it was not so strictly enforced that we couldn't sleep in the same room for most of the night.  Those who rent are liable for upholding these laws and come under heavy reprimand if they don't.  I have been renting out my additional rooms and did read (but not at length) the tenancy laws here and don't recall anything specifically regarding married couples but from what I understand, it's probably better to err on the side of caution as you don't want to be subject to the penal system here.  I would pay special attention to the response by 'Irkandarhack' the American Muslim, and do some research to verify his statement before making any decisions.

If you're Muslim the local Islamic law applies, as do the local punishments.

Should also note that you only mention that you're Muslim and I assume your girlfriend is not. That could provide some very interesting reactions from the local religious police since under Malaysian law you wouldn't be even allowed to get married.

Hopefully someone has more direct knowledge than I do regarding the rules, but the way I understand it you could be be punished under Islamic law, but your girlfriend would not be. However in the one case I that I am aware of (non-Muslim American, but Muslim Indonesian girlfriend) she was punished under Islamic law, but both had their work permits canceled and deported.

Thats an interesting point. Is there anywhere on the entry forms to Malaysia or on the MM2H application that you have to declare you religious affiliation ?

I didnt see it. So what determines you are muslim or some other species

Often your name or country of birth is the indicator to the 'religious police', however depending on what country you're from they may just leave you alone. But regardless it's a risk that I'm not sure I'd be willing to take. All it takes is one Muslim neighbor to make a phone call and next you find your apartment raided in the middle of the night and charged with khalwat or close proximity. The charges might be dropped (clearly they would be against the non-Muslim), but the hassles and attention put on you is not worth it.   

I think I remember in applying for work permits religion being part of the paperwork, don't know about MM2H. You're right in that it's NOT on the application or medical forms, which somewhat surprises me. However the target audience for MM2H are well-to-do westerners and as such unlikely to be inclined to answer that question.

In my case being married to a Malaysian Muslim I had to declare myself as Muslim to immigration in applying for the spousal visa, so there's no doubt in government paperwork that I am Muslim. Which was the case of the couple I knew before, the woman had been married to a Malay man was in the system as being Muslim, the man was an American on a work permit (teaching at a local university) - she was charged and punished for Khalway, he was not. But most important thing was that BOTH had their work permits canceled and deported.

By the way, it was noisy neighbor that reported them to the religious department, they didn't care for an American man living with his Indonesian girlfriend without being married. They thought it was bad example for their children. It wasn't that didn't like the couple, but didn't care for the example it set.

Bad idea, walking on thin ice.
I don't want anything I say to be moderated, but be warned authorities/officials in Malaysia can be very dramatic, and sadly racist too.