You don't have to adhere to any social norms in order to advance spiritually. Often it's even counter productive. In the east there were "yogis" who had many partners and it was considered a means of self-development.

Secret relationships could possibly backfire at you though. "Secret" suggests that you are not completely honest with your partners, which could invite unwanted drama in your life. I actually have a few friends who have several partners and they are up-front about it. They have attracted partners who do not insist on an one-on-one relationship. Also I was told there is a good book on the subject of "unorthodox" relationship models but I don't remember the title. Maybe you can find it if you search on amazon.

EDIT: hey I found the book. It's called Opening Up. Also there is a wealth of resources here.

The truth is you cannot have a 'secret' relationship. As much as you may want to keep it secret, the very fact that someone else is involved prevents this. You can never be certain that the other party is also interested in keeping this secret. There are only two possible reasons for wanting secrecy. One is based on a fear of criticism or judgment of others. As part of spiritual growth you should strive for immunity from others' judgments. The second is based on dishonesty. Dishonesty is counter to natural laws and also would seem to hinder spiritual growth. Remember spiritual 'success' is not a destination, but a journey. Why place self-imposed obstacles on your path?

i think karma applies in this case and it will not affect your spiritual journey my friend.respect.

I agree with eziospags. There will be consequences that will definately teach you lessons on your spiritual journey if that is what you mean by achieving spiritual success.

How and what we attract to ourselves with regards to the attainment of spiritual or any other kind of success is not based upon the act in and of itself; it is based upon how we FEEL about the act or what we are doing...

Secret Relationship? I think the karma will teach and guide you how to be succesfull in spiritual path. maybe you need to see what the good and bad in your present situation.