Helmut Lang’s show began innocently enough, his trademark sleek trousers, body-conscious tops and functional overcoats redone for Spring in nubby silk-knit burlap. The looks were pure Helmut: functional, urban and modern. But then, gradually, a wave of sexual innuendo began to escalate. A tape-like strap was strategically placed on a semitransparent top; a bikini bottom was paired with a deep-plunging tank. Finally, a procession of crisscrossed, bondage-inspired dresses and tops whizzed through the audience. Lang, the designer who pioneered androgynous, uniform dressing, had designed a brilliant collection that could go from a Tribeca loft cocktail party to an alleyway midnight rendezvous west of Times Square.