How would Elves regard infidelity?
I think this is a more complicated question than it first appears.
What we know about Elvish sexual mores comes from an essay entitled "Laws and Customs Among the Eldar " (LaCE), which appears in part 3 of Morgoth's Ring, Volume X of the History of Middle-earth series. The essay covers various topics including Elvish life-cycle, naming customs, roles of men and women, sex/marriage, and death and rebirth. The essay makes clear that the Eldar married only once in their lives, for love or at least mutual consent. Engagements could be broken, but seldom were. Although there were traditional ceremonies, vows, and whatnot, these were not necessary for a marriage to be real. "It was the act of bodily union that achieved marriage, and after which the indissoluable bond was complete." Later there's a section "Of the Severance of Marriage" which concludes that, marriage being of the fëar (spirit), bodily death does not end marriage since the spirits are still bound in "a union of the will." It took an end to the "will," when the dead are either unwilling or not permitted to return to the body, to terminate a marriage.
So according to LaCE, infidelity while both spouses were alive would be unthinkable. It's unlikely that they would be tempted to cheat: "[The Eldar] are not easily deceived by their own kind; and their spirits being masters of their bodies, they are seldom swayed by the desires of the body only, but are by nature continent and steadfast." Also, according to the essay, the Eldar are interested in sex mostly during the early years of their marriages, as sex is primarily for procreation (although it's pleasurable and fun). They lose interest in making whoopee once they are done having children.
I have no doubt that LaCE reflects Tolkien's very conservative Catholic ideals when it comes to marriage and sex. He was on the record as being strongly opposed to divorce; for him, marriage was for life. However, in-Universe, the validity of LaCE is questionable. The first draft, which forms the basis of the essay, was "authored" not by an Elf but by a Man, Eriol aka Aelfwine, who supposedly traveled to Tol Eressëa, interviewed the Noldor living there, and brought back this intelligence sometime in the 900's. So it's possible that the material in the essay was idealized by Eriol or his informants (who answers sex questions honestly?). Some of the material, such as naming practices and betrothal ceremonies, would naturally be culturally specific, and sexual mores have cultural components as well. One can at least speculate that practices may have been different for, say, the Avari or the Nandor.
Certainly, things in LotR and the published Silmarillion don't always conform to the ideals of LaCE. We have Eöl ensnaring Aredhel with enchantments and "taking her to wife." We have Celegorm and Curufin imprisoning Lúthien so that Celegorm can wed her against her will (effectively, by raping her). Maeglin tries to kill Idril's husband so that he can have her, again without her consent. On a milder note, we have several examples of Elves marrying and having children much later in life than LaCE would suggest. Celeborn and Galadriel wed (presumably) some time in the First Age, but don't seem to have reproduced until well into the second age. Elrond is over 3600 years of age at his wedding, and all three of his kids are pushing 3000 and still unmarried at the start of LotR.
As for remarriage, of course the most famous case is the second wedding of Finwë, to Indis of the Vanyar. This has been discussed in much detail. However, I think that LaCE leaves the door open for other lawful remarriages. I'd speculate that someone like Nerdanel, whose husband is doomed to remain in Mandos until the end of Time, would be allowed to remarry. I think that the spouse of an Elf who refused the summons to Mandos and remained a disembodied spirit might be permitted to remarry, since the dead spouse would effectively have refused any chance for re-embodiment. And I wonder what would have happened to Idril had Tuor not been joined to the Eldar. He would have been gone forever from the Circles of the World. Would she have been obliged to remain a widow forever, or could she have taken an Elven husband (assuming she wanted to). And what about Mithrellas wife of Imrazôr?
Reading the question another way--what do Elves think of infidelity among Men? I suspect they thought it was an unfortunate result of the Marring of Arda, but, hey, mortals--what can you say?