The answer boils down to difference between Hinayana (~beginner's) and Mahayana (~advanced practitioner's) attitude.
In beginner's mind, there is a strong conflict between Samsara of day-to-day life, and the peace of Nirvana. Buddhist practice is seen as a mean of cutting the fetters that keep reengaging one in the endless cycle of Samsara with its pointless activities and endless frustration. The challenges of day-to-day life are taken as nuisances or obstacles distracting one from study and practice.
In an advanced practitioner's mind though, Samsara is seen as an inverse projection of attachments, and Nirvana is understood as a fully integrated experience free from conflict between desirable and undesirable, not a place apart of Samsara. For such practitioner, all activity becomes dharmic practice, with glimpses of Nirvana hiding in the here and now, behind the curtain of dualistic mind.
As someone who's been married for 19 years, and a practicing Buddhist for about the same time, I can say that married life provides endless possibilities for overcoming one's pathological habits, dropping one's hang-ups, surrendering one's egoistic facades, and sacrificing one's petty personal goals in the name of the higher good. As long as one operates in the right context, basically that growth requires overcoming the ego, married life becomes the best dharma school one could ask for.
To answer your question, whether romantic relationship is a form of attachment, it again depends on one's attitude. One can take romantic relationship as a candy designed to satisfy one's urge for the sweet, and cry when it is taken away. This is the egoistic perspective. Or one can take romantic relationship as a learning opportunity: to expand and enrich one's relationships with the world, to learn to improvise the dance of life beautifully, to be open and impartial, unfettered by petty desires and hangups like jealousy etc., to stay ever joyful despite the normal challenges. And when impermanence takes over, one can take that as a learning moment as well, and accept it with sobriety and detachment. This is a perspective of the warrior. Or one can take romantic relationship as a chance to enrich the other's life, help the other overcome their wounds and neuroses, and to grow. This is a perspective of Bodhisattva.
The same applies to other forms of engagement with the world. As long as we take them as exercises helping us grow, transcend our ego, and achieve harmony and enlightenment, they are dharmic tools. The moment we make any one of them as all-important in and of itself, and place above the fundamental value of universal existence, we are in for trouble and suffering.