One possible word is "nonogamist", formed from "nonogamy" in the same manner as "bigamist" is formed from "bigamy", or "monogamist" from "monogamy".
As noted in the -ist entry in wiktionary, suffix "-ist" can be "added to words to form nouns denoting ... one who follows a principle or system of belief." As "nonogamy" refers to not marrying, a "nonogamist" is a person who chooses not to marry.
Note that "nonogamist" suitably answers the original question asked, about "someone that [sic] has consciously chosen to stay single for life", but is perhaps not suitable with respect to "relationships" mentioned in the update. Perhaps "philononogamist", which actually is quite easy to say after about 14 tries, would serve. Also, note that those in the "nonogamy.livejournal.com community" would use "nonogamist" to mean pretty much what is spelled out in the update, but their usage may differ from more common usage.
The balance of this answer treats of "nonogamy", in an effort to show that while it apparently is not a standard dictionary word, it is a word that is in use in the English language.
For an older use of "nonogamy", see the Melbourne Punch publication, page 42, 9 Feb 1871, reprinted in Mr. Punch down under, 1982. Unfortunately the author of that item wrote Section XV in a sort of summary form, rather than in complete sentences; to wit: "Children considered from different points of view. ... Expediency of not having any. ... King Herod's system spoken of approvingly. ... The author's opinion. Wives. Polygamy, monogamy, nonogamy. Nonogamy recommended. Arguments in favour of polygamy. ... Monogamy unhesitatingly condemned. The opinions of all ages and one sex dead against it." (Ellipses mine.) It appears in this context that nonogamy means not married.
In a catholica.com.au forum, Stan Kopacz wrote
"Considering the WW III nuclear landscape that marriage has become, I intend to stick with Nonogamy."
nonogamy.livejournal.com says, "This [nonogamy] community is for those of us that roll our eyes when we hear relationship stories. Those of us that don't feel we need to be paired off to be happy. We don't HATE love, but we do hate those that won't shut up about it already. Nonogamy means that we don't need love, sex,cuddling, dates, or valentine's day cards. Love is all well and good, but stop dwelling on it already."
This rgaffney blog apparently equates nonogamy with the concept, "nothing to do with marriage."
Some ignorati at democraticunderground.com make fun of the word, rather than giving it proper obeisance, in a thread called "Midthread quiz: Does "nonogamy" mean zero wives, nine wives, or wives who do "no-no"s?".
Urbandictionary offers two perhaps-ironic definitions of nonogamy somewhat at odds with meanings above:
1. An exclusive and ostensibly romantic relationship that is devoid of sex.
2. A sarcastic reference to modern monogamy in the developed world where sex is marginalized or disappears in a marriage [...]