The design is the visual features of the MULTI-FACETED THERMOPLASTIC STRAIGHT PIPE FITTING as shown in solid lines in the drawings, whether those visual features are features of shape, configuration, pattern or ornament, or any combination of those features. The portions in stippled lines do not form part of the design. The shading does not form part of the design.FIG. 1 is a top left perspective view of a MULTI-FACETED THERMOPLASTIC STRAIGHT PIPE FITTING having a hexagonal outer shell, in accordance with a first variant of the design;FIG. 2 is a bottom right perspective view of the design of Fig. 1;FIG. 3 is a top plan view of the design of Fig. 1;FIG. 4 is a bottom plan view of the design of Fig. 1;FIG. 5 is a left view of the design of Fig. 1;FIG. 6 is a right view of the design of Fig. 1;FIG. 7 is a front plan view of the design of Fig. 1;FIG. 8 is a rear plan view of the design of Fig. 1;FIG. 9 is a top left perspective view of a MULTI-FACETED THERMOPLASTIC STRAIGHT PIPE FITTING having an octagonal outer shell, in accordance with a second variant of the design;FIG. 10 is a bottom right perspective view of the design of Fig. 9;FIG. 11 is a top plan view of the design of Fig. 9;FIG. 12 is a bottom plan view of the design of Fig. 9;FIG. 13 is a left view of the design of Fig. 9;FIG. 14 is a right view of the design of Fig. 9;FIG. 15 is a front plan view of the design of Fig. 9;FIG. 16 is a rear plan view of the design of Fig. 9;FIG. 17 is a top left perspective view of a MULTI-FACETED THERMOPLASTIC STRAIGHT PIPE FITTING having a decagonal outer shell, in accordance with a third variant of the design;FIG. 18 is a bottom right perspective view of the design of Fig. 17;FIG. 19 is a top plan view of the design of Fig. 17;FIG. 20 is a bottom plan view of the design of Fig. 17;FIG. 21 is a left view of the design of Fig. 17;FIG. 22 is a right view of the design of Fig. 17;FIG. 23 is a front plan view of the design of Fig. 17;FIG. 24 is a rear plan view of the design of Fig. 17;FIG. 25 is a top left perspective view of a MULTI-FACETED THERMOPLASTIC STRAIGHT PIPE FITTING having a dodecagonal outer shell, in accordance with a fourth variant of the design;FIG. 26 is a bottom right perspective view of the design of Fig. 25;FIG. 27 is a top plan view of the design of Fig. 25;FIG. 28 is a bottom plan view of the design of Fig. 25;FIG. 29 is a left view of the design of Fig. 25;FIG. 30 is a right view of the design of Fig. 25;FIG. 31 is a front plan view of the design of Fig. 25; andFIG. 32 is a rear plan view of the design of Fig. 25.