Why does intermarriage have opposite meanings...meaning marrying within your group as required by society and also meaning marrying outside your religion or ethnic group. Confusing?

Yes, it is confusing. It's a miracle that anyone can learn the English language at all! Yes, "intermarriage" has two meanings--

"inter" as in inside--for example some religions teach their followers to marry only within their religious group. The synonym is endogamy.

"inter" as in between--for example there is a high rate of inter-marriage between Jews and non-Jews. The synonym is exogamy.