Personally, I came to the conclusion that the most logical reason behind the conception was that Mary was raped by an older male family member. Given how rape was seen back then, especially with the whole marrying your rapist laws, and how rape victims are often shamed into submission to deny the rape by their rapist it makes sense to me. Even today young girls are most likely to be raped by an uncle of other older male family member. Similarly if the rapist is a family member they would have far more control over Mary to force her to lie about the origin of her pregnancy.
I don't think the roman soldier theory is as likely, however, given the militaristic hold of Israel by the Romans at the time it wouldn't have been unreasonable for a roman soldier to abuse his power and forcibly rape Mary. This is especially possible given the general contempt the Romans had for the local people in occupied Israel. However, I stand more by my family member rape theory based on current rape statistics.
Although I can understand the Christians getting upset. The virgin conception of Mary is a corner stone of the religion and implies that Mary was pure when she conceived, as well as implying the divine nature of Jesus. If he was the child of rape it makes Jesus nothing but a bastard child with nothing remarkable about him at all. While that would fit with the Islamic interpretation, where Jesus is a mildly important prophet and not divine, it conflicts with Christianity where Jesus is the most important figure.
To me the stories in the bible feel more like a miss mash of independent stories. I don't believe some guy named Jesus did all that, however I wouldn't be surprised if they are the poorly remembered and factually incomplete memories of various people about various other figures all tacking on the label of Jesus. I find that even the most absurd stories have a nugget of truth in them, even if only at the conceptual level.
