What is the minimum requirement to be considered halachically married? I ask 'to be considered married' because I would like to know if there are also any opinions which suggest that people can be married without a marriage ceremony (although this is the prevalent custom!).
For instance, I have heard (cannot identify source) that non-Jews are considered married if they merely live together.
This issue is somewhat relevant to current debates in Israel that marriage has to be performed through certain institutions. Some secular jews in Israel (that I have met in discussion) don't want to marry under the rabbinate but still consider themselves married in a holistic sense in any case. So what is the minimal requirement for a jew to 'be considered married'?
There are, of course, many nafka minot to this if the definition of 'marriage' is more inclusive than just 'people who get married under the rabbinate'.
(Although this question is similar to this question, here I am asking for a more broad definition of acknowledgment of marriage status. Forgoing a ceremony of any sort, are there any de'ot that would consider someone married under a broader definition (albeit not l'chatchila) e.g. merely living together or 'marital' relations with kavana. Regardless of whether this couple keeps any mitzvot in any case, there are nafka minot e.g. kohanim wanting to marry someone who has been in this sort of a relationship. Also, since the gemara in kiddushin (1a) entertains the idea of 'kesef, shtar and(/or) biah', could this suggest that the system in place now (e.g. Rabbis, formal ceremony, eidim, etc.) is more of a formality (to cover all bases) and in reality a lot less is needed to be considered technically married, albeit not lechatchila)