By “in church” I assume that you mean within the Catholic Church and not within the church building. I realize that in our culture the word “lust” can be understood quite broadly, from “lusting after a chocolate bar” to simply finding another person to be sexually attractive.
But for the Catholic Church to lust is to seek sexual pleasure in another person solely for one’s own pleasure without regard for the other person. The Catechism of the Catholic Church states in # 2351: “‘Lust’ is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.”
To have sexual feelings for one’s spouse or to enjoy sexual pleasure with one’s spouse is fine and according to God’s design. But to use one’s spouse as an object of pleasure without concern for the spouse is mortally sinful.
Fr. Vincent Serpa, O.P.