。
Love and marriage are different things.
Literal
Romantic-love [and-と] marriage [topic-は] different [copula-だ].
A clean nominal predication. The conjunctive と links 恋愛 and 結婚 into a single noun-phrase topic, then は marks that topic for comment. 別 functions as a な-adjective ('separate, different'), and 別だ is its predicative form. A standard short proverb-style claim.