The core issue IMO is that there are several and very unlike ways of technically adhering to a "religion" (I will favor calling them doctrines, because I reserve the word religion for living, constructive teachings), and for various reasons people often go out of their way not to ackonwledge the rather different meanings and consequences of same.
The matter is confused, to a large extent it is so deliberately if not always maliciously.
Two common situations may help to show why.
1. Many parents simply presume or even demand their own children to automatically adhere to the same doctrine that they adopted (or, more likely, were raised into themselves).
That is very common, and neither quite as harmless as many people think nor quite as inexcusable as it may appear at first. It is not always even avoidable.
As a matter of fact, what that means exactly (including how invasive and abusive it is) will vary a lot even among siblings who happen to nominally follow the exact same doctrine. People can't help but develop and pass along some form of values and a vocabulary and set of ideas to discuss them. Those may be maintream or fringe, rigid or situational, harmonious or abusive, emphasized or dehumanizing and abusive. Many or perhaps all permutations are possible, even within a single family.
Come to think of it, this may well be the origin of the expression "lip service".
To some extent that is in fact a good thing, but there is very much a downside. A surprisingly high number of parents and relatives seem to be literally unable of accepting that actual beliefs may diverge from their expectations. All too often that actually develops into a cultural expectation for avoiding the subject entirely unless the terrain is well known to begin with. A very common situation here in Brazil, where mentioning one's beliefs uninvited is considered slightly rude.
2. Sometimes couples who want to marry are encouraged / pressured into making some form of accomodation for the doctrinary expectations of each other and their own families.
That, I am told, is a thorny matter for the Jewish People, who seem to be rather divided on the matter.
Again, this is neither quite harmless nor quite avoidable. If nothing else, there is the significant argument that sharing a measure of common doctrine is necessary for people to have the means of learning how to communicate with each other and how to deal with each other's expectations and needs. Parents-in-law, at the very least, will usually need some notion of what the newcomers to the family value and seek. I can't help but acknowledge that it is a legitimate need.
Whether that must translate into actual conversion, how reciprocal it is supposed to be, and how much accomodation should be made to genuine lack of interest and of compatibility with the doctrines is, as should be clear, not always easy to decide. Even raising the matter explicitly can be disruptive in many families.
There often a dilemma there, one that can be very difficult in some families. And it interacts strongly with the previous example, in that a family that may have settled for some protocol of handling their matters of belief may well find that protocol disturbed or even destroyed by the addition of a new person who may not easily accomodate to previous expectations.
A flip side also exists: adherence to a religion is to a large degree the effort to establish meaningful links to other people, and that includes learning values, vocabulary and protocol for expressing and developing those links. And, of course, the health of those links will correlate very strongly to how capable the people involved are of expressing, listening and being respectful to their differences of beliefs and values.
