Dear brother, the level of tolerance/acceptance of ICM/IRM is related to several factors I think. Among them are economic status, educational level, social status, etc. The higher up you go in these factors the less caste itself matters, class will matter more, so we see marriages between Hindu family and Maran. The more you go down the ladder of these factors the less tolerant people become of icm/irm....
When I was in Chennai, within my work force, I saw several inter marriages, be it caste or religion of Brahmin boys/girls. Some of the most vehement opposition came from the so called lower castes, than even from other religions. It seemed to me at that time that times were changing.
This surely has a lot to do with social acceptability. Members of the middle-class cannot afford the risk of social ostracization, whereas the rich may even think of icm/irm as a badge that differentiates them from those middle-class vulgarians. The same is probably true of well educated affluent brahmins, for them the risk of ostracization is significantly less and icm/irm may be seen as progressive.
Further, the icms/irms in brahmin families, as for as I have seen, are with boys and girls of somewhat comparable educational and economic status, I am yet to see a middle class brahmin girl going for an icm/irm with a railway porter or construction worker. If there are any such icm/irm, they surely are rare exceptions.