According to the Torah, Jews should not intermarry because their children will turn to other religions. "You shall not intermarry with them: do not give your daughters to their sons or take their daughters for your sons. For you will turn your children away from Me to worship other gods..." (Deuteronomy 7:1-3).
I think one can practice his religion even though he is married to a gentile if the gentile is tolerant and the kids of a jewish mother are jews anyway according to hallacha no matter whats the fathers religion.
According to halacha, it is forbidden to intermarry.
Furthermore, your example doesn't make any sense. You are talking about a guy practicing HIS religion, but to practice his religion would mean to marry a Jew, firstly, and also you are talking about the kids being Jewish if he marries a Jewish woman, but weren't you trying to say that the guy married a gentile woman? That would mean the kids are not Jewish.
I made a mistake. I tried to say that if a jewish woman marries a gentile guy, the kids are jewish.
The problem is, they are probably not raised Jewish, with a Jewish environment, education, focus on Torah and mitzvoth etc. This is also a problem if one marries a secular Jew. And a child is supposed to have an influence from his mother and his father. It's a partnership.
The goal is not to 'be a Jew' in a secular sense, as if that in itself is somehow an end, or superior to being born anything else, the goal is to
live as a Jew according to halacha. Having a gentile parent who is not focused on infusing the values of Judaism into the child, is obviously counterproductive. But the real reason not to intermarry is that it is forbidden by halacha. If you're asking
why the halacha forbids it, I think we can arrive at many simple rational answers to support the halacha similar to what I've stated above.